First off, I don't think "fat shaming" is ever OK. Most fat people know they're overweight and don't need to be reminded of the fact. They're either fine with it or have tried -- or are trying -- to do something about it.
I'd like to give another perspective on the problem. I'm a physical fitness nut and a former high school and college athlete, so it sounds funny how the fear of gaining weight has dogged me throughout life. I've always been in great shape but did carry more weight playing football in college (225 - 230 lbs vs current 195 lbs.). I purposely put on weight, believing it was needed to be a better player. I loaded up on carbs; mashed potatoes were my food of choice.
I wasn't overweight in the normal sense of the word. Pictures from that time show a good-looking, though younger and "rounder" me. However, I hated the extra pounds. I felt I looked like John Candy or Chris Farley, so after I finished up playing, I went on a crash diet.
However, as many can attest to, it's a constant struggle keeping it off. I exercise all the time. I "diet" all the time. I eat smaller portions. At restaurants I leave enough on my plate to feed the once-starving people of China my mother warned about. I have to suffer in agony while my wife praises how fantastic the desserts are; she can eat like a longshoreman, not gain an ounce and still look gorgeous.
Frankly, I'd love to let go and not worry about gaining some weight. However, my insecurities compel me to stay in shape. My self-image of the lean, mean machine won't let me gain an extra pound. I diet and exercise constantly. Inwardly, I fear I'll be rejected if I ever lose my looks. My wife assures me she wouldn't care if I weighed a little more, but the belief I'm desired only for my looks keeps me on the treadmill.
So there's another side to fat-shaming. In my case, it works. It does keep me ripped and toned. I'm not sure I wouldn't be more at peace with myself if fat-shaming weren't so ruthlessly effective.