On a side note: The entire sexy at any size body positivity movement really isn't healthy. It is not helpful to tell people they are still sexy when morbidly obese.
The issue with being morbidly obese is that it's massively unhealthy, limits what people can do every day, and means they'll very likely die a lot sooner than they would otherwise.
I don't think adding, "and you shouldn't wear leggings, also you're ugly" really contributes much to that in the first place, so I don't see people disagreeing as particularly unhelpful. If anything, putting the focus on looks in the first place is unhelpful.