I think Thornton is a combination of things:
1) Injury set him back a little.
2) The offensive line not holding up really hampers using his biggest strength, which is deep speed.
3) He still needs to learn some technique for getting free at the line. There have been two or three plays in particular I can remember where Mac threw to a spot that was open if Thornton got a clean release.
4) Play design isn't really doing anyone on the offense many favors, Thornton included.
That said, he's a rookie and shoulders his share of the blame. No guarantee he gets better, even if those things are corrected, but it's hard to evaluate him (or anyone on offense) in a vacuum right now.