I think it is a very plain fairy tale sort of thing. Diana was young and beautiful and plainly mistreated (well how would you describe finding out your dh slept with his mistress the night before your wedding?) Camilla, on the other hand, looks like a horse and was the other woman in question (and married herself at the time). Plus, she doesn't talk in public so we have very little idea what she is lke in reality. It is clear cut: Diana becomes the innocent and Camilla is the wicked witch.
TBH, I couldn't care less about any of it. If Charles and Camilla are in love (whatever that means!!!! - do you think he knows yet?) they should get married if they want to.