Assuming crutches work like other bludgeoning weapons, a mithril crutch would be right about useless. Mithril’s one of the lightest materials in the game and bludgeoning weapons scale based on their weight. It’s a bit of an exploit to give your Hammerer a mithril warhammer so when he goes to mete out justice to a criminal dwarf, it’s more of a light tickle than a hospitalization