I don't think Mariota is out of it, but I do think it will probably take a slip by someone "above" him to pull him back in as a main contender. While he has plenty of impressive stats (especially the 0 interceptions), I think a lot of it comes down to performance in the big games. For Mariota, his performance in the Stanford game was not good - even if the stats don't look bad, when you saw his play, you didn't come out of there thinking he was doing a great job. Compare that to Manziel against Alabama, or compare that to Winston against Clemson. These are the games most of the fans (and Heisman voters) watch, and so peoples' opinions are going to be swayed by that as much as by any statistical compilation. There's also the "what have you done lately" effect. Mariota just recently had one of his worst games. If Manziel comes out flat in either of his last two games (both of which are "big" ones), you'll see him drop out of the discussion, also.
I don't expect Manziel to actually win the Heisman again in any case, since I expect a significant number of voters are convinced that Manziel was misbehaving in the offseason and won't vote for him, or at least will downgrade him for that. If this Winston story has legs, I think that will hurt his chances, also, deservedly or not. Up until that story, I thought Winston was the favorite. So, now you're starting to see people pull folks like Petty and McCarron(!!) into the discussion, and I expect to see Mariota get mentioned as the Stanford game recedes a bit. I don't think it's because they are as impressive as Winston/Manziel, but some people want an alternative to people they see as behavior problems.
As an A&M fan, I am very biased, but the fact that Manti Teo got as many votes as he did last year, when it seemed blatantly obvious that Manziel was the best player of the year, makes me think that voters often are wanting to weight a lot of other subjective factors beyond just football performance. In this regard, I think Manziel goes in with some strikes against him, and if the Winston story doesn't die a quick death, he will, also. Unlike last year, I think the race is a lot closer between the top contenders.