I think it's naive to think that racism isn't a factor in the level of the abuse. Yes Kate was criticized but not to the extent of Megan. I remember some papers going on about how Kate was "one of us" (although she was actually middle class not working class like the papers made it sound). Her pregnancies were also positively viewed on the whole. Megan is critised to a whole new level by pretty much all news outlets. Touching her bump, just as Kate did without critism, has caused so much critism it's beyond belief.
Writing on the bananas was an Americanism and the people who are going on about it being ill judged are doing so because it's an Americanism or because they need a reason to judge her - they interviewed people who received them and they were really happy with them, plus it got the charity loads of attention thus lots more donations. The writing on the bananas was a good move for that charity and the people it helps but let's forget that and critise her because she must be in the wrong.
Racism isn't just saying "I don't like X person because he/she is black" it's about having different standards, judging them more critically than that a white british person who did the same. I do sadly believe that is what has happened with Megan.
And that isn't saying that everyone who doesn't like her is a racist - it's saying there is clearly an awful lot of extra critism from racists, as well as the people who really would critise her the same if she was white british.