Wow!! I didn't realize so many took Kay as a pain in the ass.

I put other. She was kind of all (except pain in the ass). She was a victim kind of, but got into the situation of her own free will; she was smart and loving, but IMHO, and although I can't put my finger on exactly why, she lacked warmth to an extent.
Although, I've always felt Appollonia would have been the "better" wife, the Kay character was necessary to bring that "conflict" between Kay & Michael. Appollonia would have been another mama, keeping quiet and turning her cheek to any of the family business, and would have never gotten an abortion, which is one of the most dramatic and I think important scene in the movie.
TIS