being a young player with tons of potential is at PSG of all clubs cant help. with their player-centred approach (no discipline, accepting any and all behaviour from the superstars etc) it probably will hurt (and perhaps has already hurt, given this season) his development.
i remember Sir Alex once talking in an interview about how his staff at United managed all the youngsters they signed. really preparing them for a professional life. that's the sort of nurturing players like mbappe need. Neymar's example is really bad for him
The problem is with Mbapp? himself. He wants to play like Neymar when he doesn't have the qualities of the brazilian. He needs to play much more directly, and mostly get his physical attributes back. He was known for roasting any player due to his speed but now he can't get past anyone. The whole team seems smoked though. Maybe Covid affected him a lot and he has lost a lot physically.
Anyway, although their fitness is important as high level athletes, this shows that he is way more dependent on his physical attributes than some other elite players.