It's clearly stated that she will die young due to health related issue, which is true in real life. Fatphobia is used to talk about people who hate purely on the appearance of another, which is not the case of this story. Her parents love her chubby look. She didn't want to lose weight because of how society hate her appearance, she wants to avoid death flags.
Fatphobia is not erased by someone having legitimate health issues that they need to address (in ways that would be both physically and mentally safe, gradually fixing bad habits, stable and maintainable for them). It is true that she needs treatment, it is not true that the way she talks about herself is necessary for that.
Insults and inherent negative social bias do not disappear simply because you are sick, and the way someone talks about even themselves (but esp so in circumstances like fiction where they've popped into another person's body) , can unfortunately still add to negative stereotypes and attitudes towards people. Especially so in media that often puts an emphasis on pretty and thin/dorito= amazing. That extra layer of hatred towards fat people is completely unnecessary.
Especially so when a decent amount of these stories use weight as a gigantic character flaw to be fixed along with often untrue features with it and implicit negative character assumptions. (ie the "white pig" villainess manga where she describes herself as (paraphrasing) 'i smell like i havent washed for a week after one evening of exercise and grease simply pours out of me like a cooking turkey due to extra weight'.)
That only adds to the issue of plenty of series using fat characters as a visual and character short hand for them being evil, lazy, worthless, low intelligence, etc because they are incredibly rarely portrayed in a positive light in anyway at all. (and when they are it can be just to make someone look better or still be with asides about their diet, or physical appearance, or a caricaturist assumption about what an overweight to obese person must surely be eating to be in those circumstances.
I'll save this damned family does a much better job at the weight issue than any other k series I've seen, even that still falls prey to being deeply unkind to the original occupant.
This is all i'm going to say on the issue though, I'm not planning on coming back to this particular series, I dont have anymore words to add, and if folks disagree theyre obviously free to.