So I know a lot of people like to depict Mane-iac as the pony equivalent of the joker, but I was wondering what do you all think of a character that only comes across as insane when she is acting out the role of the villain but for the most part acts and seems fairly normal (aside from the seemingly sentient mane) in her normal life?

Also what would be a good way to explain such a change?

Just do what BTAS or Word Girl did and have a villian go through a tramatic accident that affects them neurologically. That should work pretty well.

You could also give them a specific complex that they style their crimes around but are otherwise mostly normal. Those are just some suggestions though.