>>150382415
There's never been a consistent reason, and the show has given multiple answers as the situation deems fit. The one most people remember is that she has a literal bug up her ass.
Fairly Odd Parents, at least originally, is an unusual show in that you can sort of see how the characters may be in a "real" scenario, so your brain tries to fool you into thinking that's the "real" interpretation of the character. But the show itself scorns grounding and consistency, and prefers to reinvent every character as the bit demands. Vicky in "reality" is just sort of a lazy, dickhead teen that does babysitting gigs for easy money. The show has her vary from a greedy sadist to the embodiment of evil.