Tommy Hearns, as much as I love him, just didn't have the greatest set of whiskers. And particularly so at welter, where as others have noted the weight-draining really made his already-questionable punch resistance even worse.
It's already been stated, really. Most guys go up in weight for dubious reasons....can't keep their weight down, or looking for bigger fights...but Hearns is one guy who should have been at higher weights sooner than he was. A six foot two inch guy weighing in at 145...that's Karen Carpenter country.
It does make you wonder because if he had gone up at either the end of 78 or during 79 there was Koshi, Kalule, Hope, and Mattioli, holding WBA, or WBC titles and tbh I would have fancied Tommy's chances with any of them. Obviously he hung on for the big money fight with Leonard.