On encounters that reasonably approximate single target scenarios (Festergut, to an extent even DBS), I more frequently observe a Fury Warr or Combat Rogue topping the charts than a Fire Mage. These aren't players who are sub-par either.
Not really a conventional chart-topper. Unless you sit close to melee range the entire fight and trap dance, MM belongs to that 2nd tier that has potential to top, but requires either exceptional RNG, or a gear or skill advantage over the aforementioned 3 specs in the first tier. Some other specs that belong in this 2nd tier would be Feral Druids, Frost DKs, Rets, and Affliction Warlocks.