To calm you down, even though I cannot say this with a 100% certainty, I do not think being really underweight is that trendy as it was in the nineties.
People are conditioned to see underweight bodies as desirable, the natural reaction is neutrality or even pity and concern. As a thought exercise, what do you think, what goes through your head when you see an underweight animal? Most people say empathy, pity, and this is what people normally feel when they see underweight people without the added labels of: this is what models look like, or: look at that flat stomach.
In modern societies with food security, people have grown taller and larger, so I would say being thin is even rarer.
But maybe I am divorced from "beauty" trends. I would definitely say there are more thin people in big cities, and in social groups with younger people.