There's nothing wrong with what happened here, but its really abrupt and that's annoying. Wanting to become healthier and feel better about yourself isn't a bad thing, but it's "bam she's hot now and now she gets a kiss," when one of the fun things about the manga was how she was fat. Her realizing that the guy holding her was some skinny dude and just... falling on him fuckin' ruled so much
I suppose its a bit too much to want the series to slowly change the way she's drawn, but this sort of poof, pretty now is always frustrating, and it being in this sort of manga where her weight wasn't a problem for good people before and is actually one of the reasons the plot happened does feel off. That she was fat because of stress eating junk meant that this plot element was seeded a long time ago, but it never felt like something that needed to be resolved, but rather something that would resolve itself over time.