KM definitely has extensions (as well as regular blow dries), you can tell by the thickness of her ponytail when she wears it up, and just how consistently her hair looks the way it does, whatever the climate and weather. She also has veneers and fillers.
I’ve no opinion on any woman in the world doing any of this stuff. But when you’re Queen-in-waiting, this level of vanity belies a proper insecurity and misunderstanding of what your role is. Yes, she IS a clothes horse and she IS photographed a lot. But it doesn’t matter what she looks like (as long as she’s clean, neat, inoffensive). Her job is to support the king and raise the next one, publicise the charities she is associated with, and be discreet. You don’t need extensions, fillers or veneers to do any of these things!
I don’t see anything wrong with being fashionable or wearing nice gowns. But to tamper with what you’ve got to the point of extensions/veneers/fillers suggests you’re not happy with the way you are. That you want to look different, better, more something-else. It suggests you actually dislike your own appearance, to the extent of eradicating your own self to a degree. It suggests you actually care about appearances A LOT. I don’t think that’s befitting of a queen-to-be.