Of course judging people based on appearance is common. It's also common for cyclists to go the wrong way on a one way street while using a cellphone -- doesn't mean it's a good idea.
What I react to -- and I'm not saying it's relevant in this case -- is discussions that seem pointless and gratuitous. Very often a conversation about poor Sally and her weight problem simply provides an opportunity for those discussing Sally to pat themselves on the back for being superior to Sally. It's a case of building oneself up by putting other people down. For some people, this is a big part of their social behavior and a way of connecting with others.
When I was younger I enjoyed responding to older relatives with wide-eyed innocence and the question: "Why are you so interested in Sarah's weight?" Some were flustered, which I loved; others would claim to be "concerned." Well -- maybe they're concerned, maybe they're just gossiping due to their own insecurity and lack of social skills. When I was lucky, I could shift the conversation to something interesting or positive about Sarah -- and more often than not the "concerned" person had no idea what I was talking about because they had no real interest in getting to know her. They just wanted to bash her.
I've outgrown that approach, of course, but in an extreme case I might state that a conversation makes me uncomfortable, and if it's really extreme I might explain why.

