@Dragoma
a) She's agility-based. Any armor she could wear (unless magic, mithril or equivalent) would increase her defence at a cost of more agility than she would be willing to pay.
b) Teenaged. Want to look nice.
c) 2nd chance. Might as well change your entire lifestyle now that you have a new life.
d) Freedom. She might not have liked the 'stuffy clothes' she had in Japan, especially during her hospital stay, so she's happy to make her own choices now.
Take your pick. Oh and if you think all japanese are conservative, look up gals, gyarus and ganguros. Not many of them are part of isekai stories, but if entire classes or groups transfer, there tends to be a few that use their charm more and dress more... provocatively.