I think Mrs. Cameron looks fine. She is slim enough to get by with the look she’s wearing.
As for the faux pas of the lack of hat, she might be something of a rebel telling the royals they cannot dictate her fashion choices.
Okay, looked at picture again and maybe Mrs. Cameron is a little too casual and hippy looking even for the sports event.
However, she doesn’t look bad at all. I’d have to see some other pictures of her at other events to give her a final grade.
(being next to Michelle helps)