Fatshaming is almost never done out of a genuine concern for the person's health, and it's debatable how effective it is at getting people to lose weight. Especially because of how confusing the process is.

Tbh I would feel way worse and demotivated (I'm not morbidly obese but js) but especially If they're already struggling with mental issues then they'll just feel worse about themselves if they're being shamed on top of it all... They'll feel even more anxious or depressed and want to eat to fill that void, so you're really just feeding a feedback loop (depending on the person, of course) and making it worse for them. Unless I am misunderstanding that the usage of "shamed" here means constructive criticism/advice that you are genuinely giving to them. Even then, perhaps they might already know what they need to do, but they aren't making progress and lack the motivation/executive function to do it.

You don't know what their circumstances are, just try to help them out best you can and don't make them feel like shit.

Had a kid tell me "Want a cheeseburger fatty" when I was trying to walk to lose weight. Lost all motivation because of that. So if you shame fat people, you're an asshole making it worse.