I think both of her outfits are appropriate for the occasion.
The first looks was for a church ceremony. Should she have worn the purple dress there? Of course not. Church attire means knee-length dresses/skirts and no bare shoulders. Anything else wouldn't be "fresh" or "modern" but inappropriate.
For the evening party she chose another fitting outfit. The purple cocktail dress is hardly "frumpy" but very chic and youthful as are all similar dresses she has been seen in like her Boodles Boxeing Ball outfits for 2006 and 2008 and the short corale dress she wore at the Henry van Straubenzee charity event.
I don't see a middle aged couple at all and I do not get where Kate's cloths is "frumpy".