This is pretty scary, quite honestly.
I think an ability like this might toss Heracross into solid OU status.
But on Mence? That's really, really, scary.
Coupled with choice items doesn't make this great though.
The rise of Overconfident Mence's means the rise of Skarmory and Fortress, so the ability with a choice item isn't going to be game changing.
However, if egg moves do work with dream world abilities, Dragon Dance sets are going to be a shitload more scary. D: