I don't think MM is that bright. She married and seems quite happy with H. He's hardly a towering intellect. I'd still wager that she wears the trousers in that marriage.
She's also not articulate. Her unscripted sentences meander, use 57 words when 10 (or sometimes none) would do, contain poor grammar and syntax. It's very difficult to be articulate off the cuff. Stephen Fry and Giles Brandreth, for example, are very articulate. Bojo isn't. Being articulate comes from a love of and ease with the chosen language. Not everybody cares for it or can manage it. If the Prime Minister du jour can't manage it, one can hardly beat MM for it. (I think the last, properly articulate Prime Minister we had was probably John Major? Maggie? Harold Wilson?)
She also isn't well-spoken. I think people are confusing softly-spoken with well-spoken. She isn't articulate enough to be well-spoken imo.
Her scripted speeches are performed emotion. The content is feeling, not intellect. This is the background she comes from: acting, lifestyle blogging (which is parlaying emotions into the sale/purchase of things). She's good at this. She knows how to convey a specific image (the southern california, boho chic, expensively ungroomed image) very well. Not so good at conveying an unfamiliar image (eg wife in the RF).
It makes complete sense that she felt out of place in the UK, and is now happy to be home. Yoga, selective vegan-ness, a certain aesthetic, a strongly held entitlement to happiness, carefully manicured nature, expensive and hypocritical woke liberalness - she's the quintessential Californian girl. By marrying H, she's moved up a few rungs financially, but at a terrible, terrible cost to her and her family (and maybe friends). This too is, arguably, quintessentially American.
Separately, I'd be pretty confident guessing that Kate's frostiness at the Commonwealth Service was the culmination of a series of interactions/happenings. If FF is the best MM can manage about Kate, you have to wonder what went before.