When is she dismissive of Andrastianism? Dismissive of the Chantry, maybe, but of the basic religious beliefs behind it? Doesn't being dismissive of the Chantry put her right in line with the majority of the Inquisition? It's not like nobody else believes that the Chantry needed to be "fixed".
Wut.
The reason Sera isn't dismissive of Andrastianism is that she believes in Andraste. To the point that she makes fun of all other religions and people who believe in them for being "stupid". After Temple of Mythal:
"You don't believe anything from Mythal?"
"'Course not.What, I should believe all that stuff, because Elves? I mean it was impressive and all, makes the Dalish look like ****** for livin' in the woods but so what? There's can't be a bunch of gods and The Maker. Don't matter how much or little you believe those don't fit. So, call me stupid, but I believe the stuff not made up by dead people who failed; Mythal is a ruin full of demons. I mean, it just makes sense right?"
"The evidence is hard to ignore. I believe it."
"No, now you're stupid. You can't think that because it's stupid."