Absolutely. If he was mentally tougher he would have way more slams. It's funny, when he is confident and playing well he is nearly unbeatable as I said in another topic. But any doubts creep into his mind, and he chokes ridiculously. He was always a talented player who didn't have enough physical or mental strength to win big titles consistently. Then 2011 came and he was both physically and mentally tough.
Looking at the slams he lost.
U.S. open 2007- choked a lead away in one set but was always going to lose to Roger, who was just the better player.
U.S. open 2010- was having the worst year of his career and was exhausted from semi. The fact he got a set is hilarious. Nadal had no business winning it really, he was lucky it was in Djoker's roughest patch.
RG 2012- he didn't choke, he just got fucked by the rain delay. Should have won this still though.
U.S. open 2012- wind affected him a lot but that's not to discredit Andy who handled conditions better. Did well to come back and force a fifth set. He's the better player but it wasn't too poor a loss due to Andy just handling the conditions better.
Wimbledon 2013- fucking awful performance. His RG 2013 choke ruined any chance he had in this match. He was so flat and awful. Mentally one of his worst performances. It was almost like when he got a lead he apologetically handed it back.
U.S. open 2013- again, RG 2013 caused this, woeful first set before hitting form in the second. Owning Nadal in the third before literally handing the set to Nadal with UE afer UE. Nadal did nothing to earn the set or the match.
RG 2014- won the first set and then his sickness took hold. Not a choke from him but very unlucky again. A fully fit Djoker would have taken this.
RG 2015- wins the first set before wilting under the pressure. A great performance from Stan but a real missed opportunity from Novak who has a clear mental block here.
So he is 8-8 in finals, but really the record should be massively in his favour and would be without his sometimes awful mental fortitude. Often he starts to play better when he is down match point and it's too late. So yeah, Djoker should have way more slams. But that's life. Sadly he can't handle the pressure often
