First, I have no problem with Sera being in the game, and I think she, among others, brings a nice range of possible companions, and can be especially useful if you want to have an Inquisitor who is also could be loosely described as "Chaotic Neutral" like Sera.
Sera provides range for the companions, and another example of a realistic flawed character. She is a great addition to the Dragon Age Companions. Also, any judgement on her based on her speech pattern or appearance is invalid. While pretty much everything here is subjective, that is completely subjective and no character should have to 'look' or 'talk' a certain way to be 'worthy' of a spot as a companion.
That said, I absolutely detest the character. While she does have a few positive goals or ideals, her childish, selfish approach to them really ruins any ability I have to be sympathetic to her. I can keep her around, I can use her when needed, but overall I find it hard to respect her or her beliefs.
I agree with many others that she is close minded. I really don't care if she shares my Inquisitors Beliefs, but she does not respect others beliefs, and she makes judgements of others based on their social status, not their actual personality or actions. She is the reverse of an elitist. She has the same judgmental view as a high ranking noble who thinks everyone is beneath them, in reverse. Anyone 'big' is assumed to be a jerk, probably not worth leaving alive. She makes an exception, only mildly, for the Inquisitor(if noble), mainly because he/she is the only one that can fix the main problem, which she is smart enough at least to realize the scope of the problem. I don't care really that she doesn't treat the overall plot seriously, sometimes its good to take things lightly, but this disdain she has for anyone else who does take it seriously, that is what gets me. Its a little odd she is even willing to work with the noble inquisitor, as he falls into a class and makes choices that she has indicated in other situations she would kill nobles who were similar. Or any noble in general really.
She is comedic, her dialogue is often hilarious and great for a laugh, but as a person, Sera is very close minded because she doesn't respect other viewpoints, or really respect other people in general. Her dialogue, her choices, her actions, demonstrate very clearly her main focus is getting things back to normal so she can go on striking back at the upper class in the most fun and humorous ways, instead of the most effective ways for helping. Rules are bad, laws are bad, the ruling powers are bad. While many of these statements are true in many cases, she applies them indiscriminately. Again, she has the same narrow view, just in reverse, as the ruling elite that she despises.
She is happy to kill nobles or anyone 'big' without actually knowing if they are good or bad people.
She threatens and demands the Inquisitor think and believe the same as she does if she is going to be close to her.
She shows disdain for other viewpoints.
and many more.
Talking to Sera is like trying to explain rules and responsibility to a petulant 10 year old.
Again, I'm glad she is in the game, and I have no problem with some people who like her, but the way her character is designed is should not surprise anyone she is disliked as much as she is.