I think it does have an effect. I dont think any team that wins the league does so without having that mentality of overcoming setbacks. Call it what you like, attitude is probably a better word as you've put it. To be a top level footballer you have to be mentally strong. You just have to, you have to have thick skin, the ability to drown out 40k people and focus on the task at hand. It doesn't mean when it goes right you can't enjoy it, but when it's not going to plan, to have the mental strength to overcome it.
I'm not convinced we have alot of mentally strong players in the squad, not just from this season but from many. I dont think any one or two individuals could change that personally, I think it can help having strong minded people in your squad, who can help others but it really depends on the individuals and how much they can handle.
I've not felt for a long time we have a team who mentally can handle pressure. Even under Ancelotti who doesn't feel any pressure at all (based purely from our outside perspective), players looked like they could crumble.
I feel like the last time we had a squad that really had that strong mindframe as a majority was probably that era of Arteta, Jagielka, Cahill, Baines, Neville, Lescott and so on.
*side note - I also dont think we have many technically good footballers either, so that could play a huge part in the collapsing that we have seen, a lack of confidence in ability and so forth.
We get a little off topic here
@Lanolin but have to admit, bringing it back to Michael Keane, when he plays well, he seems to really play well but it's hard to imagine that he's got a strong head on him. In life that is okay but as a premier league centre half, you need to have that arrogance/strong headspace.