I think of it as the other way around. If Lumpkin defended the PnR even adequately, our guards would have been shown to be decent defenders (well, not JJ). Almost every time his man set a pick, he backed away, making it impossible for the person being picked to have a chance, no matter how they played it.
I don't know if Lumpkin should get all of the blame for his PnR defense (vs. the coaches). Once or twice early in the season he actually came out and slowed down the ball-handler (almost like he knew how you're supposed to do it). It seems to me that the coaches wanted him to back off of the pick (maybe to avoid fouling, since they seemed to have so little confidence in McCloud). Otherwise, why would he purposely avoid even trying?