What would be your Mahariel's reaction to...

It's an interesting question. I saw my Mahariel as not really believing in the Elven "Creators" but by that token, that's in part because before Inquisition, I feel like the lore didn't really suggest any Dalish were true believers, and more acknowledged the old traditions for the cultural aspect. There's also the fact that the "real" Mythal had really nothing to do with the myths the Dalish passed on anyway. So ultimately, I think my Mahariel would still see Flemeth the same way: an ancient Abomination who presented herself as a threat. That she shared a name with a creature from a fairy tale would probably be mostly immaterial. Granted, Mahariel has less reason to know everything in the Elven myths was false, but just the fact that "she's not locked up in the Fade" probably tells that as it is.

Now, that said, if we go back and retroactively apply Dalish belief to Origins and do work under that assumption...I'm not sure much would change, because again, the sheer fact that she met Mythal is proof that whatever she did believe in was wrong, so she'd probably view Flemeth the same way. If she'd been a believer, killing "Mythal" would probably be less relevant as much as learning that "Mythal" as she knew her never really existed in the first place.