Isekai de Seijo ni Natta Watashi, Genjitsu Sekai Demo Seijo Cheat de Kanzen Shouri!

Byodoin Rea's family has fallen into ruin, she's the target of bullies, and she's living the worst life ever. But one day, she journeys into a mysterious world inside a mirror and becomes a "Saint." Not only does she gain mysterious skills, but she also somehow succeeds in losing weight! She becomes a beautiful girl that everyone turns to look at...!? A major reversal in both the other world and reality with the Saint's cheat!

(Source: Kodansha, translated)