At the risk of putting words into the author's mouth, I think this story is less about being OK with being overweight or about it being bad to want to change yourself, and more about not giving in to societal beauty standards. All of the characters kept the changes to themselves that they liked. The point is that they re-examined which changes they made to themselves because they wanted to, and what they did because of societal pressures.

Kenzie did away with what society told her was sexy and kept only what she herself needed to feel sexy.

Rod stopped caring about having to appear masculine and instead found a way to live out his power fantasy as a more authentic version of himself and have the long hair he always wanted.

It's implied that Zedd only gave his monstrous form a washboard because he felt he was supposed to. Perhaps he is an artist who learned to draw by imitating DBZ or something in a similar style.

Finally, Melissa is written as though she only feels that her freckles, her glasses and her figure are ugly because she has been told so by others - bullies, influencers, society in general. When she takes a step back to consider how she herself feels, she realizes that she likes these parts of herself.