I found this photo at fanpop.com that was posted a year ago and I think it’s clever and amusing! The only similarity between Cinderella and the Duchess is, of course, the fairytale wedding. Kate Middleton was no Cinderella when she married Prince William, having come from a well-to-do family.
As for the famous hats of Princesses Beatrice and Eugenie during the royal wedding- they are, no doubt, better than those of the wicked sisters in terms of weirdness (or creativity, if you wish to call it that).
But when it comes to British society, she indeed is Cinderella. Her family might be wealthy, but they are neither royalties nor do they have peerage.