interestingasfuck

How about, "it's not socially acceptable to mock, or otherwise comment on others physical form unless invited to do so by that person."

But that makes for poor sound bite/click bait.

The body positive movement for the overweight was a reaction to a long tradition of a large number of people just thinking it's ok to openly mock "fat people" because "it's their fault for being lazy and ugly, and we should just tell them all the time in case they don't already know, also it's funny and makes me feel better about my own personal failures that are less outwardly obvious". But you probably know that already.