According to the Heisman website, here’s the guidance for how the trophy is awarded:
The Heisman Memorial Trophy annually recognizes the outstanding college football player whose performance best exhibits the pursuit of excellence with integrity. Winners epitomize great ability combined with diligence, perseverance, and hard work.
Tyrann Mathieu is a fantastic football player. There is no doubt about that. But he was suspended for a game this year for violating his team’s drug policy. Obviously, he’s disqualified for the award.
In my opinion, any vote for Mathieu is a wasted vote; one cast simply out of ignorance or defiance.