Perhaps the perception of her 'unseemly' behavior is due to misunderstandings. It could be that she is trying to fit in a new social circle and is overdoing it. For instance, if she is new to a group that has a more liberal style, she might be adjusting in the wrong way. The key is to communicate clearly with her about what is acceptable and what isn't.
Well, the spoilers for a manga like this could involve plot twists like finding a unique diet method that works wonders or facing setbacks that make the journey more complicated. Maybe there's a surprise friendship that helps with the diet too.