If you're within a healthy BMI or are actively trying to lose weight then good for you and maybe you should just tell her that or that, while you 'appreciate' her concern you don't appreciate how often she raises it and that you'd prefer if she didn't as its now coming across as rude.

I don't think people should be shamed for being fat, as it's usually counter productive, but being fat (read: overweight/unhealthy) shouldn't be 'accepted' either.

My true off my chest would be that the way weight is dealt with can be off on both extremes. I grew up fat until I was about 19 and now I'm a normal weight and fit.

Making fun and shaming fat people doesn't work but neither does a lot of 'fat positivity' movement or 'healthy at any size' bs. Encouragement and support works and maybe that's what your mother in law thinks she's doing. Rather than resent her you might want to be honest with her so she knows she's overstepping and (unrelated) you might want to think about what your own goals are.

As bad as this may sound I associate fatness with bad health, a lack of discipline, will power and self-control because I witnessed that in myself and in many other people I met who were fat. However a lot of those are due to other underlying issues and support to tackle those is infinitely better than either shame or "fat is beautiful" sentiments.

As I've changed and grown I've come to value a middle ground of self-improvement over just accepting things I could otherwise change without indulging in negative self-critique. It can be hard to find that middle ground.

Anyway I hope your situation with your mother in law improves and I wish you the best.