I can understand why Mawile is a Fairy-type. Its probably because of how they have a docile and cute look. I am very sure that Mawile is based off a Japanese Yokai called the Futakuchi-Onna (The Woman with Two Mouth), because of its appearence and behavior. Its Dex entry states that it tricks foes by acting sweet, but then eats them brutally afterwards, just like the said Yokai's behavior. They also look like the Futakuchi-Onna, by having one mouth on their face and the other on their hair(?).
However, I don't understand it's Steel typing. Wouldn't it make more sense if it's a Dark or Ghost-type, knowing that it is based on Yokais which are basically monsters/spirits?