Wait since when is mithril a thing in DF. Adamantine would certainly be a bad idea because crutches are bound to be blunt damage and adamantine (inspired by mithril) has about the density of cork. For blunt weapons you want gold or platinum, but you’d need to edit the game files or farm moods. Silver is the next best thing that can be done without cheesing. Adamantine is for armor and edge weapons.

The unreasonable effectiveness of crutches might be due to the same factor as the unreasonable effectiveness of pickaxes: Those use the mining skill for damage bonuses, crutches might use crutch-walker you’d have to test that. In any case wielding an axe one-handed is probably not a good idea, there’s even dorfs that aren’t tall enough to even wield them one-handed and the ones large enough to do so will still be way better at it with two hands.