I think it makes considerably more sense than hair color. Some teams pick based on factors that have nothing to do with football, such as the Bucs who said their
#1 pick this year would be based partially on "community relations." And do you doubt an owner like Woody would not put undue influence to pick a Heisman winner, for nothing more than business reasons? And then there's the possibility that winning the Heisman may affect the player's self-evaluation, where possibly he does not think he needs to work so hard to learn the pro game.
All of these are reasonable explanations of a real effect, and are light years beyond any explanation like hair color.