Fat people are literally the only group where it’s just OK to make fun of them, in many circles.
Even for those who are overweight and unhealthy, berating and shaming someone is never going to be a way to help someone heal.
Why is obesity becoming more common when every doctor tells people to lose weight then?
I recommend reading the book, the entire point of which is about shifting one's health efforts from an exclusive focus on weight to a more holistic view.
I.e. even if you can't drop that last 20 pounds (and many people struggle to do so, especially as they age), you should still eat a healthy diet, exercise, limit alcohol intake and avoid smoking.
Obese people are discriminated against in healthcare, employment, and social settings.
Furthermore, numerous studies have show that fat shaming does not lead to weight loss. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6565398/
Shockingly, it turns out if you tell someone with 100 lbs to lose that they are a disgusting lazy piece of shit doomed to an early grave, they tend to withdraw from health care and other social settings, seek comfort in unhealthy habits, and suffer from anxiety and depression. They may give up on their health entirely. After all, if you've struggled your entire life to lose weight and made little progress, then what's the point?
Now I'm not saying we need to shame anyone, but saying "healthy at any size" is a terrible lie and in the long term helps no one.