"How does Lizzo’s personal choices about her life and her body affect you in any way and why do you think you have the right to say nasty things to her about them?"
It's never that simple. Lizzo isn't just "someone else." Lizzo is a celebrity, and many people, including kids, may get the idea from Lizzo that it's healthy to eat as much shit as they like, because the result is that they will be "beautiful" like Lizzo. Obesity IN MOST PEOPLE is unhealthy. If Lizzo's metabolism makes controlling her weight impractical, it may still be unhealthy, and she should not "own it" in a way that encourages, or even justifies, indifference to one's weight.
The other day, I heard Sen. Ron Johnson make exactly the same argument about COVID prevention. If you're vaccinated, he asks rhetorically, what's it to you that others are not doing anything to avoid getting sick? Substitute COVID for obesity, and I'll bet a lot of pro-Lizzo types come out in a different place.
COVID is, of course, different from obesity. But most of the differences go away if you are vaccinated. The only difference that remains is that dangerous, vaccine-resistant, variants may arise in people who have the disease, so universal vaccination and caution protects the herd from incubator patients. But otherwise, the problems of obesity and COVID in others are similar. Both raise the cost of healthcare for everyone, and both put stress on the healthcare system. And claims by an influential person that it is OK to contract either condition because you aren't hurting anyone else set a bad example.
The argument that "most factors that contribute to it are not within a person's control" strikes me as disingenuous. The important question is whether ANY factor that is essential to one's obesity is within the person's control. Some people can handle carbs better than others. Insulin resistance is a factor "not within a person's control." But avoiding carbs is within our control and it's not that big a hardship. And if it is a big hardship, treatment is available. You don't get a pass on setting an unhealthy example just because being healthy is harder for you than for someone else.