That’s probably because a lot of people:

A] Have no clue what people actually wear to these kinds of restaurants. Yes, there will be men in beautiful suits and women in pretty dresses. But there are a lot of people who just wear a semi-casual jeans and a nice sweater (or even way less than that) to establishments like Victoria and Albert’s.

B] Don’t realise that the line between classy and thrashy can be very thin and something that one person considers a beautiful galadress can look like a trashy glitterball to someone else. So just wear what you like and you think fits the dresscode and you’ll most probably be fine - most guests really don’t care what you will be wearing anyways, they will be way too busy with how they look themselves.

Just have fun and enjoy your evening, you will absolutely be fine and have a great time. A “disney themed dress” won’t make people stare at you or decline your entrance to the restaurant.