Is using Custom Skins not allowed?

Technically speaking you aren't allowed to use custom skins but because it doesn't change the game in any shape or form for anybody but you (and even then it's not changing much) Riot doesn't really care to much about people using them.

I don't think it's an issue since it doesn't effect gameplay like a script does. And you your a script kiddie, then you don't deserve to play this game if you aren't going to put the effort into getting good.