I haven't as yet read any real criticism of Kate ever deciding to be an at home mum (a huge an very important job IMO). I think the issue for some is that she did little whilst she was young (well, younger) unmarried and seemingly very able. I suspect it isn't what many of us would encourage our own daughters to do. Wait for a man the way Kate did. Or is it OK if it is such a man as a prince? Or a man of great fortune (heck, any fortune)?
The other contentious issue would be funding of lifestyle. Whilst she has not ever been reported of being a beneficiary of government assistance, she would, I think, have had to be dependent on someone else's income until she married. I don't imagine most part-time positions would afford her lifestyle. For many, at ages beyond the teenage stage, this is not seen as...well, decent. By that I mean, if one has a good education one has the responsibility to stop living off one's parents, or whomever. Perhaps that is not how she was raised and it is likely many are not likewise raised and find not actual issue with the way Kate lived. But I think a great many of us do and so we have the criticism you speak of. Although I understand some may be driven by jealousy, I don't accept all would be thus motivated (by jealousy). To many of us it is the common decency of making a contribution to self, family, community. Especially if one is give such great advantages and opportunities as Kate has had.
It is a lack of understanding for those who do not have the same mindset. I guess we may think those who do not seek to be independent and enabling themselves to start contribution ASAP as being selfish or lazy or both.
None of us wish any woman to forgo their freedom of choice, but we may disagree or criticize the choices made. Perhaps man are more likely to stay quiet in a social set, but certainly not in the political arena! In Kate's case, she is a public figure and many wish us to look up to her as a great example. This motivates many to make their opinions heard, especially so if they don't think her the ideal model they want to look up to (or even less so their children).
If you like Kate and don't find fault with her, that is fine and you should say it out loud but understand that those who do not can have their opinion stated too, and just as loudly. Now to have it all out in a cordial and respectful manner...that is the great challenge.