The difference is that I'm not expecting, or even wishing that Sera would share my opinion, it's the manner in which she disagrees with it that I dislike about her character (and I suspect is the reason for many other people as well).
What it really comes down to is the interaction between two people, real or virtual, and how those people respond to each other. For me, I find it extremely off-putting when someone can't give me the respect of at least listening to what I have to say, instead of just immediately dismissing it and calling my thoughts stupid. It's completely fine that we disagree, life would be boring if everyone thought in the same way, but I'd much rather the person say to me "well, I don't agree, but let's agree to disagree" because that shows a much greater level of maturity, in my opinion, than just dismissing my ideas as stupid which is often what children do.
I also don't think that Sera matters less because of how she expresses herself, because like I said, I do find some of her conversations interesting, but it is hard to make the effort of finding those interesting conversations underneath all the fart noises and 'dangle-bags'. This is honestly probably why the people who think she 'matters less' feel that way about her. Not everyone is going to like how Sera expresses herself, and as a result won't want to deal with her antics to get to know her character better.
As for your points about Blackwall, I would agree that it is a very strange double standard (but then again, I'm not an overly big fan of Blackwall either, although I would rather talk to him than Sera.)
I would be repeating myself to say it's dangerous to value form so much over substance. The typical Bioware fan/player/developer is pretty obsessed with form. Substantively, actually, Dragon Age as a whole is pretty air dried, characters and so on scream at the top of their lungs, and those judgment scenes, so over the top and showcasing the characters or developers independence and zeal, and yet they are actually reliant on the player's input to decide the core acts. Sera sticks out in this sea of passivity, it's absolutely of no surprise whatsoever that people would find her problematic.
But you know speaking of exaggeration, she doesn't come across to me as this excessive loudmouth cartoon, she seems pretty funny and biting, not just like dumb cartoon silly, in conversation and all that.
In fact, I can turn this around, why in the blue blazes don't more people criticize Isabella for her lack of attention to form or agreement to disagree? I really could not stand Isabella, she's the perfect grade A stereotype of a walking in your face personality. How the frik don't people dislike her for her in-eloquence. She doesn't make the fart sounds but I'd say she's actually considerably less attentive to form than Sera is overall.
I suspect the hatred of Sera may be more complicated. It's quite possibly she pays little heed to the form but is kind of obstinate about it. Someone like Isabella seems more easily manipulated so her unwillingness to address larger issues is considered more acceptable.