You have a point but are way too harsh and straight-forward about it. People gain weight for different reasons. Maybe they're in a deep depression, maybe they have a health condition, etc. Calling them fat forgets and that they're disgusting and worthless isn't going to motivate them in any way, no matter how much you think it will. Saying "Living a healthy life can improve physical and mental issues dramatically" is much better at motivating than "get off of the couch fatty and go run a mile".
You're looking at the surface and not the reason why they're like that. Sure, there are many fat people who are like that just because they eat too much, but don't assume everyone is like that. You act like humans don't have feelings, and saying things like this can't hurt them emotionally.
By saying "fat people are just fat because they eat too much and don't exercise" is extremely ignorant on your part, as I assume you've never had weight issues or have studied it.