The fabric of her dress was/is exquisite.
The cut and style theoretically not bad, but what kills it is the ridiculous royal code of decorum.
The cut of her top is extremely unflattering for her. She apparently had quite nice (gosh can't bring myself to write QEII and Boobs in the same post
and while she doesn't necessarily have to flaunt it on her wedding day, a nicer top could have looked a lot more flattering.
The lead in the skirt drags it down (heavens forbit the idea of the skirt flying up
So in order to cover her up completely and still be somewhat stylish the whole thing drowns her.