Cheating at D&D is in many ways just a very bizzare thing to do, but given human nature it should not be entirely surprising that it happens. In D&D there is no "winning" as such in the traditional sense, but the success of your character DOES have some emotional significance for players.
It isn't as if players win real money, or real-world prizes, or actual fame, but nobody rolls up a character WANTING to see them underperform, fail, and ignominiously die. Some players are going to have... issues. Those issues are with the fate of their character being tied with their own real-world sense of worth and importance in the eyes of others. In that sense, yes, it CAN be serious enough to wander into the territory of mental health. They will cheat in an attempt to make themselves look superior through the prism of their PC. They cheat to be "better" at D&D, more successful, than the others sitting at the table.
I really can't fathom any other reason to cheat at D&D than feeling that your self-worth is endangered or your manhood impugned if your PC does badly, or dies, or simply isn't superior to everyone else's PC's. If anyone can think of some OTHER explanation why someone would cheat at D&D I'd love to hear it. But AT BEST I think that makes it outrageously pathetic. But it also makes it seem not a whit less tolerable to me than cheating at actual gambling, business dealings, or the like. Those would at least have material rewards for getting away with cheating, whereas with D&D your cheating really isn't going to destroy the game - it simply spits in the faces of everyone else at the table who DOESN'T cheat - and despite seeing PC's do badly and even die still manage to enjoy playing. There's LESS justification for cheating at a game which YOU DON'T EVER "WIN" ANYWAY.
Nope. I personally will not tolerate players cheating at D&D. I won't stand for a DM who tolerates players cheating either. If it's obvious enough to be noticeable by everyone at the table it's truly outrageous. I wouldn't care to debate the matter, to plumb the depths of someones motivations for doing it. I'd just want it stopped immediately and be able to carry on without even needing to mention it again.