Why did Cat's personality change so drastically

[TMC]

In Season 1, she seemed quite normal. She was a bit of an airhead, but decent. She would often have fits of rage whenever someone corrected her about something. But Season 2 and on, we saw a different Cat. Her voice had changed drastically and she'd often be ditsy and forgetful and say things that really made no sense at all.

TV Tropes calls this act "Flanderization"

The act of taking a single (often minor) action or trait of a character within a work and exaggerating it more and more over time until it completely consumes the character. Most always, the trait/action becomes completely outlandish and it becomes their defining characteristic. Sitcoms and Sitcom characters are particularly susceptible to this, as are peripheral characters in shows with long runs.

In Cat's case, they flanderized her childishness and naivety to the point where it was cartoonishly annoying.