It's not bad to be fat
It objectively is and it's a terrible comparison to being short.
Being fat is terrible for your health and will limit your chances in love. It's not politically correct to admit it, but most people simply aren't attracted to people that are overweight, and whilst appearances shouldn't matter, they absolutely do.
The difference between being short and being fat is that you can't control your height. Controlling your weight might not be fun, but you can essentially choose what weight you are.
Bullying is obviously never ok, but if you're not ok with the idea of being called fat, then you can choose not to consistently over eat for long periods of time.
Inb4: I understand some people have severe health and mental issues that are the primary source of their obesity and I empathise hugely with these people, but most overweight people simply lack will power. Losing weight is like giving up smoking, you can want to do it, but you don't until you say "enough is enough" and actually mean it.