It’s not like she’s cutting herself or becoming a heroin addict in order to impress her crush.
I really want to know what kind of person would be impressed by either of those things
I think you missed the point entirely. Blastaar will correct me if I am wrong, but what he was likely driving at was that she is not doing anything harmful or even that radical to herself, and thus her choices hardly merit to be called "problematic". Getting a haircut is completely normal, and adopting a moderate workout regimen is actually beneficial for her health.
Bingo. And thanks.
And furthermore, the implicit idea in the term “problematic” that a person should “be who they are” rather than change themselves according to the standards of others is undercut (to some extent) by the fact that one major job of teenagers is to find out “who they are,” which inevitably entails experimentation with style, personal affect, etc.
The original invocation of “body dysmorphia” ignored the fundamental idea behind that concept: that the dysmorphic person finds that their attitude toward their body or physical features causes them distress and hinders their functioning in the world.
I doubt that I’m unique in being able to think off the top of my head of a half dozen people, in high school and later, who undertook major makeovers (physical and otherwise) and seemed considerably happier for it. And, to be sure, a couple who changed for the worse (hence my cutting and addiction examples).
To return to the story, we’ve seen more than one example of yuri stories where girls felt forced into the “prince” role in high school because of their physique or natural affect; this may turn out to be one of those. And there is a somewhat restricting attitude about gender roles when girls are praised for being “manly,” etc. But I don’t think the concept of “body dysmorphia” gets us anywhere near what’s happening in this story, at least so far.