No, there's not a curse, but it seems pretty clear that just because a QB wins a Heisman doesn't mean a damn thing about their prospects in the NFL. It's not that they suddenly suck either, they have sucked all along (as far as NFL standards for QBs have gone), and simply won an award in college. Tim Tebow is the ONLY Heisman winning QB to have a playoff victory out of the last 16 Heisman winning QBs....that's not just a coincidence it would seem. Seriously though, think about that stat for a minute....not one QB who has won a Heisman, outside of Tebow (who couldn't even be an actual NFL QB for more than a few games), since 1987 has even won ONE playoff game in the NFL!!!
I like Mariota, and hopefully he's great in the NFL....but to suggest that a team would lose on purpose so that a division rival wouldn't be able to draft someone (let alone a QB), is absolutely ludicrous. Most NFL coaches are fighting to simply stay in the game, there aren't too many who are planning 5-10 years in the future worrying about a possible QB their rival may draft. I could understand if you said that upper management may want the team to lose, but Sean Payton is not telling his team to lose, and would scoff at the suggestion if the GM or owner mentioned it to him.