I would not say that Mbappe - Messi - Neymar performance was poor as they had combined 90 scorer points in Ligue 1. I guess it will be hard to find another attacking trio with 90 scorer points in a season in the league only. That's without Coup de France and without the Champions League. But they have still lost too many games and I think conceded too many goals as a top team.
They made it not by themselves but by cooperating with other team mates. They scored, won and lost together with their team mates. They are biggest star players in football and in their club last season but they did not achieve anything in a single match or a season only by themselves without support from team mates.
I read some news that those star players were not happy in PSG, Messi and Neymar already left. Even Mbappe who was forced to stay in PSG, looks like is not happy in this season too. PSG have something wrong to deeply reconsider about themselves as they must know if they fail to provide their players best environment, best mood, they will not be able to do best contributions for the whole team.
In the game against Bayern Munich in the Champions League round of 16 PSG almost didn't have a chance despite all the star players. The difference in their performances was unexpected and must have also been disappointing for everyone who was looking forward to PSG delivering some nice games. I guess this season won't be any different.
That match sucks because star players of PSG look like youth players and suddenly became harmless when facing with experienced Bayern Munich. Their mentality was not improved but actually became worse after their final match in Champions League four seasons ago.