how is it that Cat valentine's voice sorta changed during the show ?

Since TV actors work under strict contracts, it's most likely the director's choice (and/or maybe the writers) rather than the actor herself. Ariana probably didn't get a say in what she wanted her character to be like.

Cat wasn't originally that dumb. That was something that progressed over time. Perhaps once her character had been dumbed down a lot, the director decided that he wanted Ariana to play Cat in a ditzier way, and she found she could do this through her voice?

I definitely think that her voice changing is due to her character changing.

the directors told her to on sam & cat I think because they don't want people to know Idk OR either she retired from victorious for a while and forgot how to do the voice as cat in sam & cat don't mind the words on the pic

yeah

I think that she did it on purpose thinking that it suits her bubbily personality