Here's the problem(s) with Shelton-
A. In order to be a good babyface, he needs to have a decent ability to talk. As a heel he also needs to do this, but you can at least give him a mouthpiece if he can't do it himself.
B. His wrestling ability and style makes him a natural babyface, but he botches a lot of spots. Plus he has a better understanding of how to work as a face than he does a heel, as Cabbageboy alluded to.
C. His natural aura lends to being a heel (think Floyd Mayweather but to a much lesser extent). He has a cockiness to himself that you'd want to boo him (or cheer him if that's something you like, as evident by some MVP fans around here)
So to sum it up, he has some major deficiencies as either a babyface or heel. And this is why until he fixes some or all of the problems I mentioned, he's not going to take off as much as he has the potential to.