My reading of it was that the character having those thoughts and describing her that way was offensive, and if he changes and becomes more understanding by the end of the story, then it would work. As you say that's the point of the story!

Fat shaming is awful - the things people say out loud are horrendous. I was a fat kid, and I wrote a children's book about it (The Too-tight Tutu) - when I do school visits and talk about the book, I always see teachers cringe but the kids totally get it. The teasing, the way you are looked at, the assumption you're going to be bad at whatever you do. They know sugar coating it is not going to make it go away.