(Here I go again with a Wall-o-text reply. Apologies in advance: apparently a character flaw on my part...)
Ah, OK. Now I see the issue with Bianca. Yeah, as you went through point by point I was reminded of each aspect as it happened, feeling the same reaction I'd had at the time, and I have to agree. The storyline was good- ok, I think. But Bianca was supposed to be legendary- or at least somehow live up to or justify the hype of Varric's reverence through two games in the series. We should probably know right away that the hooded figure he was talking with in Skyhold was someone special, but it wasn't until her name came up that I even recognized that was supposed to be her- just some regular, hard-edged carta type. She didn't have to have her soul inside the crossbow or be Superdwarfgirl, just be worthy of the intrigue that Varric's loyal affection warranted. In that way, yeah, pretty disappointing. The biggest points I agree with are:
1. The story was really short. So I guess that was it then after all? I was sort of assuming it would continue given how abruptly it ended. It was like two extra fights (past the usual spawns in Valammar) and a conversation or two. That's got to be the shortest companion quest in DAI. Aveline's ridiculous romance quest in DA2 went on 10 times longer. So that certainly made it underwhelming.
2. Better craftsperson than Branka? That did feel gratuitous, but, as you said about Bianca's forgetability, I suppose I just ignored it, figuring it was boasting that would later be clarified. But actually Varric did validate it and call her a would-be paragon, so apparently I'm supposed to take it seriously. It belittles the whole Branka encounter for her to be that much better a smith. And she didn't say she was Branka's equal. She said she was waaay better. I mean, Branka was considered a paragon because she was an exceptionally rare genius- and she more or less seemed like the type despite her insanity. My Duster became a paragon for saving the world! (OK, just for a decade, but still...) If Bianca really had some genius invention, they'd have given the prize to her- at least if she were submitting it to dwarven society rather than exploiting its application on the surface, since in that case why bother awarding her anything from Orzammar? But it wasn't at all believable. "Lazy writing," as you say. Seemed like an ability awarded to her in a conspicuously casual manner- the way a mediocre action movie might introduce a character as a super-assassin, expecting the audience to believe it despite the poor acting and direction and no real martial arts abilities to demonstrate on-screen. And usually a character is as snooty as Bianca was because they're compensating for not being as good as they claim they are, not because they actually are it.
3. She wasn't particularly likeable. As Remki says, she was OK as a character in general. She just didn't exude any more character than, say, a Dagna... and at least Dagna is likeable. Bianca isn't as much of a bumbler as Tallis was, looking like a complete incompetant. I really didn't like that character. But she was written and directed to be voice-acted as snooty, overconfident, and somewhat full of herself to a fault- qualities easier to ascribe to a caricature than a character. Loyal or no, it's not particularly believable that Varric would ever have fallen in love with her. If they were to have written it as if she once upon a time wasn't such a snoot, but success or some cruel decision made her a harder person, OK, but it was "just like old times" when Varric and she spoke, so I have no reason to believe she'd ever been different. It's one thing to say Varric is somewhat masochistic, but another to say he'd revere someone so stuck on herself. Even the simple barmaid girl Felsi was more likeable and almost believable as a potential Varric love interest. Falling in love with Bianca's sort just wasn't... meaningful
Other things I don't mind so much.
1. The end quip about feeding my Inky her eyes- it just seemed like she was demonstrating a strong shared bond there and that she was still fiercely loyal to him as well. There was a hint in it that she felt deeply hurt about something she'd done to him as well- albeit also not explained. Liking Varric, I can empathize with the sentiment she expressed and felt glad she had his back as I would like anyone having Varric's back. It obviously wasn't necessary as a warning to us Varric supporters, but if you think of all the ways an Inq can be a douche to Varric (his approval does drop for certain things), the line could have direct relevance when said to an Inquisitor.
2. The lost love aspect. Even if they made her an unlikely love interest of Varric, they didn't elaborate on the reasons why they broke up (or never were together). So it still could be a much deeper story- like the guy she's with now was Varric's best friend and yet they'd fallen in love only after the best friend and she were an item but Varric hadn't wanted to hurt him by breaking them up so he left... or the guy was her ex-husband who she'd thought was dead but returned from some Deep Roads expedition only after Varric and she had fallen in love and thus Varric left to not interfere. Or maybe she was the same kind of snoot back then but they'd gotten trapped in some cave-in or something for a year and they'd fallen in love as a result. That sort of thing could even explain his "subdued" behavior. Or they could come up with something lame, alas, but since it's still up in the air what actually happened between them, I feel like it's better to reserve judgment. Given her character, it doesn't seem likely, but taken as a sticking point, I'll let it pass. If it were clear she just "stood him up at the altar" (or worse outright cheated on him), it'd be easy enough to dismiss her on that ground, but when he mentions that she needs to get back to someone and she responds pleadingly with "Varric..." as if to explain, it seems like it wasn't just some trite "she done me wrong" story- i.e., there is some explanation. That said, the way the Harding "romance" abruptly ends before it gets to "the next level" shows the devs didn't extend the theme of foreshortened relationship story development with dwarves to just Varric.
3. Bianca being a better shot than Varric. He's not that great a shot- and neither is his crossbow. (And, hell, he dropped from Lvl25 to Lvl1 when my Inq first meets him.) And it just works for why he'd name his crossbow after her rather than, say, a tattoo or armband or ring or something. You're right though- it's not exactly the most creative explanation. But at least it doesn't clash with the experience we have of her like the idea of her being an expert marksperson, an expert carta wheeler-dealer, an expert red lyrium investigator, AND a paragon inventor... 'cause she finds the time for it all. Leave out the inventor part (and two more) and I'll believe the marksperson bit. Or perhaps better, leave out the marksperson bit and make the would-be paragon bit involve siege engines- another possible link to his affection for a crossbow. But the all-too-brief storyline has her involved in everything (rather than just one part of a more elaborate story involving an arcanist, a paragon, a marksman, etc.) so they made her an unbelievable everything. Like you said, they don't appear to have put a lot of thought into it.
4. Her "less than apologetic" stance about her screw up on tipping off Cory forces to red lyrium. This part of the story I actually liked. What she said about wanting to fix her mess seemed sincere. Plus it's not as if she screwed up due to a character flaw. How was she to know that a Gray Warden of all sorts would be an agent of some Tevinter cult helping to end the world with an ancient Magister who breached the Veil? It's possible she was a pathetic judge of charcacter, but did anyone's Inky suspect Solas was what he was before Trespasser? It seems a bit unfair to hate her for that. No, she didn't listen to Varric's warnings about red lyrium's danger, but why would she? She never even heard from him after Kirkwall. Wait, they hadn't spoken since his red lyrium experience, but they said he warned her about it- oops, poor writing again. But she really wasn't an expert, now was she? And yet due to her efforts, she introduced a pretty heavy lore component that fits nicely with the whole Titan reveal: lyrium is alive. And, like the Inquisitor said, she had to know that her costly error would be made obvious once she brought them there, so it's not as if she were trying to squirm out of it. Presumably she betrayed her carta- and thus livelihood- by bringing the Inq and Varric to slaughter them. I believed she was sorry. With her not being the deepest of characters, it wasn't the most meaningful of sorry's, but, again, she did seem sincere.
5. That Varric would "stoop" to liking someone like her. This doesn't refer to Bianca's character (which is simply a sort of juxtaposition to Varric's 2-game character development), but rather just the idea of seeming to be with someone not as good as he is, even someone who doesn't have his best interests at heart. Sure, Varric deserves better, but this is something that happens. I've done this... so when I see Varric possibly having done it, it doesn't look alien to me. Not happy with it, but if Varric has the same tendency, it means the story makes him a character I can identify with, and I can appreciate that. The problem with it was more a matter of the devs not investing much into what he saw in her or why he fell in love with her, what the bond was about or, like, any story at all. It was just presented as a given, and, yeah, as such it didn't rise beyond low expectations as a typical (for DA) dwarf romance- i.e., Oghren and Felsi insulting each other 'cause ain't they cute. Rascally drunkard Oghren was never the seasoned rogue, close friend of the Champion, Defeator of Knight-Commander Meredith, and accomplished author that Varric was. And Varric has a tad more wit. Even if Bianca were plain as a doorknob, at least make her the type of doorknob who would appreciate him.
Not sure if I hate Bianca now that I've reconsidered it, but I do share the same disappointment.