Hmm, I think the matter is a rather subjective one. Nonetheless, I do like these little arguments.
From MY point of view, one should never look towards a duchess's chest area and it is unfortunate that this little rule of mine has had to be broken yet again on account of what has been said. Accordingly, I think the outfit is very lovely, a little low cut maybe, but whilst HRH is standing up, I do not think there is anything too amiss about it.
I think the outfit would look vulgar on an older woman such as the Queen or my mother, and it would be vulgar of Catherine to deliberately flaunt herself by pushing her chest out whilst pulling down the front of the garment to retrieve a stray cheese&onion crisp. Other than that, the outfit is quite suitable to allow air-flow in warmer climates.
Surely we should be up in arms about the fact that William is constantly togged up neater than a nun's wimple and never looks vulgar even in the summer.....