Is it a good idea to make a college essay about cats?
Anything that you are passionate about and shows personal growth or writing finesse is fine.
Your essay needs to be about YOU. That is really the point of the essay. But if you can somehow show something unique about you that is related to cats, then I think it could work.
Agree with intparent. You’re writing for your admission review, not an open writing assignment in hs or for a contest. Adcoms wantt to see in you the qualities they want for their community. Loving animals, how you don’t want to leave your cat and go off to college, or hinting you don’t need humans would be risky.