Why is he wasting himself exactly?
He became World Cup winner (with a pretty important contribution), best player of the year in L1, best scorer of L1, won Kopa trophy, 4th place in Ballon d'Or, a world star (after 6 months at Monaco, he was more like a Joao Felix prospect) while playing for PSG.
It is not like he has been regressing. He has (and still is) growing as a player, played different positions, found its way to become a leader in a "star-studded" team. And let's not forget the guy is not even 21. In comparison, some could say a guy like Foden is wasting himself at City because the club doesn't offer him enough playing time.
The mere fact he is actually doing very well in CL stats wise at his age shows he is not being wasted. I would even say that despite his evident talent, he needs to show up when it matters this time. He did a good job last year at Old Trafford but completely fucked up in the return leg while he had a big opportunity to seal the tie. It is where a player of his kind should shine. This is what had made Cristiano Ronaldo so relevant despite having to be compared with Messi: being clutch in the biggest football clubs competition in the world. So far, Neymar has shown to be the top dog of PSG when things get tough in CL (hence the tragedy of his injury before knockouts phase) and Mbappé more anonymous. He has to lift that barrier before he can be seen as having outgrown PSG.