It's clearly implied in the manga that she was a normal plain girl, not the outgoing gal type. This mangas writing is not the best out there but it's certainly also not "very poor", might want to reevaluate your judgement for good/poor writing.
It's pretty dang good and wholesome to me. Sorry but these dudes saying ho*s can't change etc, all that stuff sounds like something an insecure teenage boys would say. Grow up, socialize with real humans more and try to understand that humans are complex and have their own stories.