I agree that Mayhem is underrated, but I think that Bisping might have one of the most underrated ground games in the UFC. He's not going to be pulling off any triangles or anything, but he's got very good ground defense. I've never seen Bisping in any trouble in the ground except for the first round of the Matt Hamill fight. Since then he's drastically improved his TDD and ground game.
I still don't think he would compete with guys like Sonnen, Weidman, and Okami on the mat, but I think he can get by Mayhem just fine.