In the early 70s at Primary School, they read us Fattipuffs and Thiniffers. Hard to believe today. When one of the boys suggested Karen would be a Fattipuff, all hell broke loose. So I think idea of “fat” as an insult has long been understood and not a word to use casually if you don’t mean it as such. Thus in writing, probably only suitable for a loathsome character.