She isn't a princess yet, so IMO she can wear what she likes as long as it constitutes clothes.
I see nothing wrong with the outfit, I think it looks nice and suits her beautifully. However it isn't, IMO, appropriate for a wedding wear the bride looks so radiant.
I agree; it's a nice outfit but not for a wedding.
Also, Harry's trousers seem much too long, as if he's stepping on them.