Being fit is a struggle, not being fat is just eating real food in reasonable sizes. I don't agree with harassing strangers in public no matter what I might personally think of the way they're living their lives. But no, if a teenger's biggest struggle in life is not being a glutton than I don't have a lot of sympathy for them. An eight year old might have valid complaints about struggling with weight. A teenager is old enough that there's no excuse for not being able to take some responsibility for the most basic aspects of their life. In particular if we're talking about teenagers who are well off enough to have access to social media. If someone can complain about life on twitter than they can also load up a nutrition calculator.