It's not really about the skill of Mbappe however he was a good player in PSG but it's about the time he spent there as a superstar player. Since he was playing there for years many strategies they used were based on this player and the team worked with this player for a long time but now since he left PSG, this can definitely damage the team and make PSG feel this gap in the players.
They will surely try hiring some other player to fill the place of Mbappe in PSG but I think there won't be any player for it and as Enrique said they should fill his place with teamwork.
That's the difference between PSG and Real Madrid. For PSG he was a player that they built the whole team around and he was their superstar and they basically needed him for everything. Whereas, Real already won the league and the UCL without him just this season, adding him is a great thing but at the same time we are talking about Real not really "needing" him, sure it's great to have him, but if he is injured then they could still technically win it as well, so it is not a must thing for them.
PSG will become a better team after him, not because he wasn't good, he was their best player for many years, and won many things thanks to him, but now that they won't have a superstar, they will be forced to rely on each other instead of a single person.