In real life I have never seen or heard anyone say anything bad about people who are thin. When people are scary and obviously unhealthy thin it often gets a comment.
I have seen and heard people tear into people who are a bit chubby though, so I'm not sure it really applies in real life.
I think comments of 'too thin' on things like adverts are people just getting angry that the only people ever portrayed in fashion and the media are a particular shape, ie thin. People don't like the fact that this is presented to them as what "women should look like", and so they get annoyed and say they don't like it. Similarly, it is assumed that is what all men want in a women and what all women want to look like - people are vocal in saying they don't partically want that because they feel its being forced down their throats.
I don't believe they really believe you are ugly, inferior, or too thin. But they don't want your body type presented as a "perfect" aspiration that everyone else needs to starve themselves to get.