When Kaeloo transforms and tries to beat Stumpy up, he remains unfazed and uses his newfound intelligence to make notes on her psychological condition. She remarks that he is "scaring her". At first, it may seem that she's freaked out by his unnatural transformation from a moron to a genius, but given Kaeloo's Control Freak tendencies, there's also the possibility that she is scared because he isn't afraid to cross her anymore.
Mr. Cat wanting to be a dentist actually makes sense when you consider that it is a job that requires intelligence but also causes pain to people; it's the perfect job for an evil genius like Mr. Cat.
