Okay, that last page with the bird thing at the bottom. From what it looked like, it was actually slightly scrawnier than the one in the mirror.
So, as far as we can tell, those reflections aren't actually people who wanted to look inhuman, but ghosts who already looked inhuman but coincidentally also were going to the gym to get buff just like everyone else. Which is dumb but hilarious and completely in-line with the tone.