Your oversimplifying. I agree I don’t think being overweight should be necessarily seen as a good thing, but that’s still someone’s personal business. They may be trying to loose weight. They may have a medical condition or an addiction to food. They may just not have access to good, healthy food, or are uneducated about how to eat healthy in the first place. Making fun of them is useless at best and hurtful at worst, because chances are, they know they’re overweight. They don’t need you to point that out to them.
Even if someone chooses to be overweight, making fun of them for it can still hurt others. By all means, say that it’s unhealthy and that it shouldn’t be normalized but don’t take it out on individuals, especially if you don’t know their situation.