I think KM’s dress wouldn’t look so bad on a woman with a fuller figure, even if the skirt part is something Camilla would wear (it’d look dreary even as a net curtain).
Like a lot of what KM wears, it’s fine in theory. She just doesn’t dress for the body she has. She sees things on paper and says “that ticks the decent/modest/not shocking boxes, I’ll have two of those please, one in pink and one in mint green”.
That’s why she always looks better in skinny jeans - they suit her body so well, wouldn’t look as good on a fuller figure.
Also, even the keenest of clothes horses must get fed up thinking about clothes and hair after a while. Maybe she just doesn’t care so much any more, just wants to not have to think about it anymore. Only spends time on her evening wear.