I was with you until about half way through that. Yes, many people are going to be averse to her liberal, anarchosocialist, flippant-if-not-iconoclastic world view. Conservative, traditional, leaning to some degree towards authoritarianism, somewhat other-directed perhaps, such people are pretty mainstream in today's environment. Sera is going to get right up their nose in short order.
...and that's where we part company.
Sera does not take other people's faiths or views lightly. If she didn't take them seriously she wouldn't argue so passionately about them. What she does not do is necessarily treat those beliefs with respect...but why should she? Beliefs are just ideas, and some of them are vacuous, or evil. What she doesn't do is give lip service to "open mindedness" when she encounters such wrong... and why should she? After all, it's not like the conservative, religious, authoritarian folks that she offends are particularly open minded, "not very open minded" is what "conservative" means.
Beliefs are just ideas for people who've never needed to believe in something greater than themselves in order to put up with a harsh reality. You live everyday with violence right next to your door and you come to take faith in God or some higher power very seriously.
But to a school or college student, faith is an academic matter. And of course it would be, since they are usually protected from the worst reality has to offer in many ways.
Beliefs are not just ideas. Even those who create beliefs believe deep down in some unchangeable truth. Everyone has something they hold sacred, above the rest. That might not be religion, but it's a personal faith. And mocking that the way the character does shows she has a very shallow spirit.
Especially saying the Herald would sound stupid for admitting the myth of Mythal may be true. Why stupid? If she's so intelligent, how can she not see that believing in a religion or faith does not mean you can't respect and consider the religion of someone else valid and true as well?
You ask her why is it impossible for some of the belief in Mythal to be true. A revolutionary and open-minded person -which is what the character is SUPPOSED to be- would admit that might be so, which would make elven hystory something neither the Dalish, nor humans be right about it. And that a new hystory must be written.
Instead, she says they were arseholes like everyone else. And because they worshipped demons, that made them even more pathetic. Where's the broad-minded revolutionary there?
Sorry, but she sounds as someone who still holds a grudge against the lady that brought her up, the orlesian society, who always pushed people like her around and is now fixated in one single view of reality.
She's fanatical. Better yet, close-minded. Anyone who disagrees with her, even without invalidating her argument, is either an arsehole or pathetic. That's not being anti-conservationism. That's just being childish, immature and stupid.
No wonder so many people quickly come to hate her, no matter how "funny" and "accessible" she is. She's so accessible she can't live with a simple divergence of opinion.