It's hardly a new idea, but I think Grace Kelly is as beautiful as women get. It's hard for me to believe that any woman ever alive or yet to be born could top her out. Her arms are simply the eternal feminine ideal. There are sexier women, certainly--Blake Lively, Victoria's Secret "Angels" and Maxim covers, Liz Hurley--but such overt sexiness is not the feminine ideal.
John thinks she's "too perfect," but what can such a phrase mean? If you find a flaw, then she isn't perfect. I think what people generally mean by "too perfect" is too cookie-cutter, too all-American, or too polished and pristine. I assume the third option is what is applied to Grace Kelly, because she's so beautiful she's not sexy. And I assume this is why John does not prefer her; he prefers a visibly seedy underbelly, a la Jennifer Jason Leigh.
I think I find Grace Kelly beautiful partly because I imagine she was not perfect in person, IRL. I think she so often played a feisty (but vulnerable) bitch because she was one. If Grace Kelly's brand of bitch were a candy it'd be a York peppermint patty: shiny wrapper, hard outer shell, cool but soft and yielding interior. What option has one to be a bit of a bitch when one is so beautiful? And what option has one, when one is too beautiful to be sexy, but to be, in the parlance of her time, a raging slut? IRL, she was.