He is a great athlete, sometimes the misconception of him as "not a top athlete" comes from the fact that he isn't the fastest WR out there but he isn't that bad at around a 4.5 40 time. Just from watching him muscle the balls away from DB's and giving the effort to grind out extra yards, you know he's a great athlete and a tough guy.
The knock I've read about him on the local TB papers is that he simply hasn't been putting in the work to be a top WR. I'm not sure if that's film, practice, or weight room. Of course anybody that saw him play both ways at LSU knows he's a great athlete. That's nothing special in the NFL though.