Disclaimer: I am not a fan of MMA and think that most (not all) of those who follow and/or participate in it are douchebags.
I kind of view MMA the same way I view US soccer. Our best athletes do not play soccer (they play football and basketball) and when we go up against teams from Europe and South America, where the best athletes do play soccer, we get trounced. MMA is pretty new still and I don't think it attracts a high caliber of athlete. With equal training I bet the best athletes from other major sports (and also the best boxers) could handily beat the current crop of MMA athletes. As (or if) MMA continues to grow this will likely change.