Poor Tallis. She was loved just a little too much, and like a spoiled child, she found acceptance outside her loving family circle was not as easy as it would have been with more realistic expectations.
I've learned a lot about Tallis since MotA came out, and what I've learned has softened a vitriolic and intense dislike to merely a reaction against poor execution. Her biggest enemy turned out to be her writers. Felicia Day did not write Tallis, nor did she design her. According to Matt Rhodes, she was there from the initial design stages, perhaps even earlier than Merrill or Anders. As a character concept, she may have predated Redemption rather than the other way around.
As a concept, she's pretty cool. We don't get a chance to interact with too many elves who find refuge in the Qun, so that's kinda nifty... but the character development started at the end instead of the beginning. In other words, it made too many assumptions. It assumed that everyone would automatically be in love with her from the word go, so she came in at her end-stage cockiness and just assumed that the player would buy it about a character he or she had never met. Her entrance is similar to Varric's and Isabela where she displays her competence and flair right from the outset. Nothing too out of the ordinary there, all of the characters are shown at their best in their intros... except that Tallis's took it too far. In Varric's the first thing he does after shooting the thief and restoring Hawke's worldly goods is declare "You're the boss!" and he keeps that up throughout the entirety of their relationship. Isabela goes a bit further with the forced flirting from Hawke (which might be out of character, depending), and for a long time, that rankled and I had a hard time learning to love Isabela (but I eventually did, and very much so). But even then, the fight was her own, and she was the object of the attack. She didn't ride in guns ablazing and save a suddenly inept Hawke from getting his or her backside handed to him or her while Hawke stood around mouth breathing. I'm sorry, but you really have to earn that kind of a cutscene, and Tallis hadn't. Heck, even in Captured!, the one time in DAO when the protagonist was truly helpless, the player has the choice of handling things him or herself or sitting back and waiting for a rescue. And if they're good enough to beat Ser Cauthrien, the whole thing can be avoided. Tallis takes away that player agency, and we're forced to sit there and watch while our characters are turned into simpletons. That doesn't leave a good impression.
And it doesn't get better from there. Throughout the entire DLC, no opportunity is overlooked to diminish the skills and abilities of Hawke and his or her followers to make Tallis look good. She's better informed about Kirkwall's underworld than Varric; she's better at both potions and compassion than Anders, the party healer; she knows more about intrigue than Leliana; she's more universally desired than Isabela; she speaks better Tevene than Fenris, and pokes fun at him for his accent when speaking qunlat. The only thing she doesn't do is rip Carver's greatsword off his back and slaughter Baron Arlange's forces singlehandedly (I have to babysit her constantly so she doesn't die constantly as it is). I get it. She's supposed to be badass. But they didn't have to try that hard, nor did they have to make the players' longtime friends and allies look like morons in the process. She could have at least acknowledged their competence, which she never does. Again, you have to earn that kind of a spotlight, and Tallis never did. It was just assumed that the player would love her enough to overlook the deliberate belittlement of Hawke's friends and allies. And that doesn't even begin to touch upon the business with the lock.
One of the reasons people love Captured! is the funny banter while the Warden's allies try to free him or her, and yeah, they got that part down. But it's important to note that the Warden (and Alistair) can get out of the cell on their own, and during the course of the rescue, the Warden's companions (who the Warden has grown to know and love throughout the entire game) are allowed to show their competence along the way. In MotA, the banter is there, but both Hawke and his or her companions are once again made to look like idiots for Tallis's benefit. Even though Hawke may well be better at picking locks than Tallis. Once again, that kind of spotlight has to be earned, and the player has known Tallis for about an hour at that point, and it doesn't sit well.
But Inquisition isn't DA2, and we don't know in what capacity Tallis will appear, if she appears at all. It's possible that she'll be an adversary, in which case all of her previous detractions become assets. There's a certain satisfaction to beating a skilled opponent, so that might cast Tallis in a better light. Or, if she does wind up being a temporary companion at some point, there's the chance that the writers may have learned that most people don't like being side-kicks in their own story. They don't like it when people make fun of their friends, people who have already established themselves in the protagonist's affection. They may allow her to shine as a gem among many without making her look like a plastic rhinestone with a platinum setting. A sky with only one star isn't much of a vista.
So I'd say it wasn't Tallis herself that was the problem, it was the execution.