I think a lot of the negative comments, particularly in things like the Daily Mail, aren't from the UK. It's a site marketed to the US and Australia as well and, like it or not, the issue of racism is far more polarised in both. To many/most British people, a dual heritage woman marrying a white man and having a baby isn't anything special. And where people are racist, they are less likely to be vocal about it.
Saying that, however, there are undoubtedly still festering pools of racism in the UK as well. Perhaps if comments from overseas (and there is no way of telling, after all, the site allows posters to be anonymous) legitimise those comments, you then end up with them snowballing. Papers play to their audiences.
There is an overlap between sexism and racism in that people tend towards having similar attitudes - a racist person is more likely to feel that a woman is supposed to fulfill their idea of traditional roles.
As it is, all Royal wives (and girlfriends - and husbands/boyfriends, to be fair) attract a considerable amount of prejudice and criticism. The difference is when a new one comes on the scene, the previously vilified one usually becomes elevated.
Prince Philip has always been criticised for his heritage. Princess Margaret's suitors were criticised. Prince and Princess Michael of Kent were regularly ripped to shreds. Then it was the turn of the next generation - Diana was slaughtered, then beatified, then slaughtered, then deified. Sarah was treated as a breath of fresh air and then torn apart. Her daughters are routinely slagged off for weight, their looks, their choice in men. Sophie was a gold digger and scam artist and a beard until she was considerate enough to nearly die in childbirth. Kate was a nice girl, then a calculated harridan acting under instructions of her mother, then became acceptable for conforming to the image of a Wife and Mother, particularly once Harry's girlfriends became of interest - then they were the target for their sexual behaviour, their appearances, everything.
Unfortunately for Meghan, she has a car crash family that just keeps on giving. And she will always trigger people who hate the thought of interracial marriages. And she doesn't conform to the notion of an obedient, absolutely perfect wife. I actually like the fact that she had a visibly larger build after the baby was born. It's rare for somebody in the public eye to be seen as anything other than physically perfect (or at least Photoshopped if not tummy tucked). Not everybody snaps straight back, not everybody wants to. But I like that she didn't - it possibly explains to some extent why photos were more carefully managed - who wants to have the papers calling them a fat cow; it already happened in the previous generation - nobody wants to be the next Fat Wife, as that's something which would never, ever go away.
She's no better or worse than any other person. She's just a person. But, as the press won't get as many clicks from 'Oh, how lovely' stories from the demographic that would normally like Royalty related stories, they go for the ones that get people clicking to be shocked and angered.