This is one of the best ever shoujo manga that fits your description. It is 40 chapters long with beautiful art and it is about a pair of twins- with one being boring and the other being pretty. A story about how the boring twin will emerge into a beautiful butterfly. is definitely worth a read! http://www.mangago.zone/read-manga/papillon/ ヾ(☆▽☆)
Oh! There is this other shoujo manga about four boys trying to turn this shut in girl into a beautiful lady. http://www.mangago.zone/read-manga/perfect_girl_evolution/ ╮(￣▽￣)╭
how about this :DDD
http://www.mangago.zone/read-manga/watashi_ga_motete_dousunda/
but i'd say this is mostly comedy ヾ(☆▽☆)
ヾ(❀╹◡╹)ﾉ~ can you recommend me mangas where the girl changes from boring to gorgeous? I'd like it not to be a one shot but that's still fine.