But they sell Messi, Neymar, Verratti and Ramos are the sign they're not really want to win the Champions League. They only sign mediocre players in this season and they didn't sign a top coach, so I don't expect they will go anywhere in this season.
PSG sold those players because they failed to win the UCL trophy. They are also old players, PSG wants younger players to rebuild their squad. The current players of PSG aren't mediocre players, you must know that there are many top players there. And the current coach is Luis Enrique, he was the coach of Barcelona. Luis Enrique already won varied trophies with Barcelona and he ever won the best coach award in 2014-2015. So, you're wrong if you said PSG didn't have a top coach. 
Mental could also become one of the factor, since they can't become a champion, it might make them think they're not really deserved in this league and they feel inferior.
Indeed. PSG players have no winning mentality in UCL competition. They often almost reaches the big matches but they failed to win the match because of weak mentality. PSG needs a coach who can improve the mentality of the players. Luis Enrique is a great coach, I think PSG already hired the right coach for this season.