Not sure where this idea that Guy was a mediocre athlete came from. I'll give you that he needs to get stronger, and that might be his biggest issue early on, but his athleticism is fine. He posted excellent numbers at the combine in the shuttle and the lateral quickness drill. You can put whatever value you want on those drills, but their intent is to determine the athletic ability of each player.
Lane Agility - Shuttle - Sprint - Standing Vertical - Max Vertical
Kyle Guy: 10.48 - 3.07 - 3.27 - 30.5" - 36.5"
Terrence Davis: 10.69 - 2.97 - 3.28 - 29.5" - 34.0"
Kevin Porter: 10.82 - 3.03 - 3.14 - 27.0" - 34.0"
Nassir Little: 12.15 - 3.10 - 3.24 - 31.0" - 38.5"
All three of these players are considered by the pundits to be good athlete's, or above average athlete's. As you can see, Guy more than holds his own against all three of them. Personally, I go more on the eye test, and having watched Guy play for the last three years, especially the last two, I certainly wouldn't call him a mediocre athlete.
What I do know about him is that he plays with a chip on his shoulder, he can shoot the hell out of the ball, and he give 100% effort at all times. Lastly, he's a winner. How all that translates to the NBA remains to be seen. I can't guarantee his success, but I wouldn't bet against him either. He is exactly the kind of player that succeeds in the NBA. It might take him a couple of years to work out all the wrinkles, so some patience will be required. That's something hard to find around here.