I think 2011 was a spectacular year and one for the ages but we cannot use that to judge the rest of Novak's career. So discounting that stellar year, Novak's mental edge is just one that comes from being really damn good, having won a few slams, knowing he can beat anyone on any given day, and being a solid consistent year-round strong world number 1. And no, he hasn't lost that.
He clearly has his preferred surfaces, and he has his good days and not so good days, like anyone else. The only players who are true tests/threats to him anywhere are Nadal and Murray, and maybe Roger. For instance, as good as DelPotro played in the semis, DelPotro was always playing to stay in the match and push Novak while Djokovic was always the favorite to win, mentally and it was just a matter of time. That's the mental edge we're talking about. Come hardcourt season, given the season so far, Andy is the only one who might be able to say he stands an equal chance to beat Djokovic, against everyone else, he'll have the mental edge cuz he's the man to beat.
And yeah, let's not forget, as Herc pointed out, he's still #1 by a LARGE margin, has 6 slams and much more to his name, is still the most consistently strong player this year across all surfaces and will almost certainly end the year #1. No he has not lost the mental edge. Just a not so great day at the office on a surface that is not preferred against an opponent and a crowd that was on a mission to make history.