R121 - Kate has her couture stuff altered, too. Meghan could have done the same thing - for reasons unknown, she chose not to.
That said, there probably are a good many casual outfits Kate gets off the rack that she doesn't need to have altered. She is, after all, tall, slim, and athletic. Her only real problem is a long torso, but you don't necessarily need to alter everything to take account of that if the clothes are selected well. She's got a decent up-top, a cute arse, nice legs, and a small waist. She's also 5'9". Kate could get away with stuff Meghan couldn't at about 5'4", no waist, a low bosom, a barrel-shaped middle, and skinny, shapeless legs.
Kate sensibly ensures that whether the clothes are or aren't altered, and some aren't especially the really casual stuff, that they emphasise her height, legs, and waist, and fit. I don't like everything she wears, but mostly she seems to nail the "look" she wants to define her: nicely turned out, clean lines, feminine, and once in awhile with the very short skirts, nicely flirty.
Meghan never nailed down a look - she was all over the place. I'm quite sure she could have, but I suspect she doesn't take advice well, and went her own way, with utterly disastrous results like the de la Renta dress she wore to the Spencer wedding. Even the hugely glammed up stuff didn't really work if you looked closely, like the bras outlined under the grey Mouret dress she wore in Ireland, and that Alexis Carrington green number she wore on her very last outing with the BRF at the Abbey.
Few women are perfectly proportioned - Diana had a flat, boring midsection and no waist to speak of. But she worked with people at VOGUE who persuaded her to shift the emphasis to those incredible legs. She wasn't a perfect beauty, either, with the large nose and nondescript mouth. She had four terrific attributes: her beautiful eyes, beautiful skin, her height, and her legs. They added up to more than the sum of their parts.
Meghan just never figured out the formula to do the same, and get the attributes to work together to be greater than the sum of the parts.