I do think C does well as Princess of Wales. Now, she in no way had a middle class "non-aristocratic" upbringing. Her boarding fees for one of the most exclusive schools in the country were paid from her father's trust fund. That's upper class, not middle class. And while she might not have had a title in front of her name, she would have been exposed to all the unwritten rules and behaviour of the upper classes of British society from a very young age. Meghan had no such head start.
That said, even the most aristocratic wither and rot in the microscopic view of the British media, public, and the internal royal expectations. I think she made her peace with the restrictions very early on. I mean, she very likely has had to put up with William's infidelity, just as the Queen put up with Prince Philip's. I read Baroness Glenconner's book (a great read) and she explained how, in the very upper classes, it's just completely accepted that your husband (and sometimes wife) will have affairs. You don't leave each other because of it. Look at Camilla, her husband knew all about her affair for years. That's got to be tough, on top of all the restrictions on how you must look, speak, dress, dress your children etc etc. You can tell that she lost weight post-engagement, and has been underweight ever since. That's never a good sign.
There's only a very small cohort of even the upper echelons of society who'll put up with that. And to be perfectly honest, there probably has to be something a little askew in the head that you would be willing to put up with it. The money and the Hello covers just wouldn't be worth it for a normal, well-adjusted person. I would think that she has some chronic self esteem issues, and that being William's wife artificially fills that hole.
Having said that, just because normal well-adjusted people wouldn't put up with it doesn't mean Meghan and Harry are normal or well adjusted. While Meghan absolutely did not deserve the lambasting she got from the press - which likely did have an element of racism in it - she's clearly a fame-hungry idiot. If you were trying to make a mess of your reputation and brand, you couldn't possibly have made the amount of missteps that pair have made.
But just because Meghan's an idiot, doesn't make Catherine some kind of paragon of virtue. They both have their problems.