On the topic of Bianca being a Mary Sue; compared to who, exactly?
As far as I can tell, Bianca isn't a Mary Sue compared to anyone in the game because there aren't any others from what I've seen. A Mary Sue is a character who can do basically anything and get away with it, and everyone loves her. People dislike Bianca because it feels like BioWare expected us all to love her by giving her all these skills and witty remarks and trying to make her an independent dwarf who don't need no Inquisitor, but she also caused a lot of deaths. I mean, a LOT of deaths. Also thanks to her, red lyrium is now pretty much everywhere because of her mistake, and it screws up a lot of things. Despite all that, it seems like everyone gets over it quickly, and she's portrayed as being the greatest thing ever, even though in a realistic setting yelling at her would have been perfectly within the Inquisitor's rights. In fact, given that we can judge all sorts of people as Inquisitor for lesser things than what she did, she could have conceivably been tried for assisting Corypheus and the corruption of the Red Templars/Mages and the proliferation of the red lyrium. And yet the game sweeps all her wrongdoing under the rug, like it expects us to go "No big deal." That's how she's a Mary Sue.
Morrigan is a magical wunderkind.
Morrigan is also about as sociable as a cactus. If she were unsociable and super skilled but adored by everyone, then she would be a Mary Sue. But because her ability is tempered by her prickly personality and questionable actions, she is not a Mary Sue.
Leliana is an amazingly adept spymaster/spy/assassin/Divine
But she also is fallible as a human. She is shown to have several crisis of conscious and faith as a result of her actions and the deaths of those around her. She questions and doubts, both of which show that despite her great skill, she is as damaged as the best of them. Her struggles to maintain her faith and humanity keep her from being a Mary Sue, because she is not portrayed as infallible or impervious to the struggles of living as a spy.
Zevran's been waging a one man war against the Crows cuz he's just that good
But he was also taught by the people that he's fighting against. At the height of his career with the Crows, he was one of their top agents along with Rinna and Taliesen. He knows all their tricks, their habits, their methods, their locations, everything that a person would need to take them down from the inside. And he's not just fighting them on his home turf, they've chased him from Denerim to Kirkwall, and he spent a lot of time basically running and killing the ones coming after him. He has killed one guild master, and therefore one branch of the Crows, but there are others out there as well. By the end of it, they basically leave him alone. He doesn't (and probably couldn't) kill them all.
And besides that, he's also not a Mary Sue because he's made severe mistakes, such as letting his first love get killed in front of his eyes on false charges, and after realizing this he became suicidal. His attempt to kill the Wardens was a suicide run, and he expected to die (which he says in his companion conversations). Not everyone loves him or even likes him, and despite his charming personality, he's still a murderer. You don't have to like him. Hell, you can even kill him if you like. So he is not a Mary Sue because he has also failed and doesn't expect everyone to love him even though he flirts with everyone.
Isabela can manage to steal the tome of Koslun twice
Although she managed to steal it from Seheron the first time, it is implied that it was almost sheer luck that she manged to survive, and even that was because she deliberately sailed her ship into a hurricane knowing that the dreadnought chasing her probably wouldn't survive. It was nothing short of a miracle that she got out of that one, and she also only managed to steal it the second time because Hawke was helping her, she wasn't sneaking into a land controlled by Qunari, and Kirkwall itself was a din of chaos and anarchy which also made the job easier.
Isabela isn't a Mary Sue because, like Zevran, she's a take-it-or-leave it kind of person. She's made mistakes but she works through them, and she's not interested in getting close to people because they tie her down. Her actions are not excused by her companions, and it's pretty clear that if she felt that she really had to, she would ditch town in a heartbeat. The only thing keeping her back is loyalty if she's close to Hawke.
Hell, Varric's readership spans the entirety of Thedas, he's a (reluctant) successful businessman, good/excellent archer, dabbles in being a spymaster and now rules Kirkwall. Honestly.
The readership thing is actually pretty realistic. Books can be translated, printed, and transported pretty much everywhere, so it makes sense that he's known all over Thedas. Hell, the Sherlock Holmes novels were even read in Japan when Sir Doyle was writing them! He's also a dwarf raised in a merchant class family where he was not only expected to know everything about business, that was his life. That was literally what his family lived for. Dwarven caste systems are pretty rigid, even on the surface. Also, he's involved in the Carta, so skills at being a spymaster and being a good shot make sense given his background.
And Varric is also not a Mary Sue because he has some pretty significant flaws. He doesn't think about the future, he lives partially in the past, he's hung up on a woman who lives hundreds of miles away who is married to another man and whom he gets to see maybe once or twice a decade, and he's a liar. You can call him out on being a liar at least, and that particular trait wins him very few friends. People may like him, sure, but no one really knows or trusts him aside from a select few.
My point is, ultimately, that all these other characters have flaws and problems and issues and the game doesn't behave like we're supposed to like them. We're allowed to react to them either positively or negatively, and it reacts back accordingly. We don't have that option with Bianca. We can either love her, or we can love her. Even the rudest, most aggressive Inquisitor can't really do anything to show their disapproval. She basically gets away with her idiocy scot-free, and it makes no sense why that is. Why can we allow Celene, the empress of Orlais to be assassinated, and yet we can't smack a dwarf in the face for spreading blighted lyrium around that drives people insane?
THAT is why people don't like her.