Yes it is harmful to the people themselves. Then help them with kindness. Not by shitting on them and bullying and offending them. A lot of people are overweight because they are depressed. And just because there are a few people that think its healthy, doesn't mean they all should be made fun of. Talk to those specific people directly instead of being a bully towards everyone.

I don't see people being hateful like this towards people smoking.

Don't be a dick. Its not that hard.