Shojo manhwa cliche?

I don’t get why there are tons of shojo mangas/manhwas that have tan mls yet a lot of these same manhwas also have tan mc deemed as ugly but once they turn white they suddenly become ‘pure’ and ‘beautiful’ and ‘lovable
Also genuine question cuz I didn’t watch or read one piece, why/how did robin go from tan to white? It’s a genuine question and in no way trying to attack anyone, I’m simply lacking information and seeking it