No I understood perfectly because there was very little evidence about ancient elves and their gods. But now we know that they were actually immortal and elven gods did exist. Now she is being stupid or she has a very low IQ.
We know something god-like existed, but the game is very vague about what they were exactly. As for being immortal, well Mythal did get murdered. A piece of her took residence in Flemeth seeking revenge, but it's difficult to know if the elven gods were spirits, mages or something else entirely.
As for Sera's reaction, I would attribute it more to denial than outright stupidity. She is scared of the Fade, spirits and magic. It's easier for her to cover her eyes and ears and go 'lalalala' than ponder the implications of those beings actually existing/having existed. The elven gods did exist, and most of them are gone. Are they dead? Are they trapped like the Dalish believe? If they are either of those things, what made them disappear? Is it still around? Could it turn against mortals?
They were gods and they were wiped out. What chance do mortals stand if that happened?
About Sera being uneducated, well yeah. She is. She didn't exactly live a privileged life. She still joins the Inquisition so the world will make sense again. Her motives can be a little selfish (get things back to normal so she can play again, etc.), but she still tries.