Paris SG squad is more than enough for League 1. We also saw that Mbappe was rested in some matches because League 1 does not put much pressure on them. Naturally, they will focus entirely on the UCL rather than the league. Mbappe is a great player and has made great contributions to Paris. I am sure that he will continue to contribute to UCL. I think he will go to Real Madrid and do wonders. Mbappe is an important signature player with his contributions no matter which team he goes to.
The Ligue 1 for PSG is more like a stress free league for them where they get to play as they like and probably chooses when to loose a game and when to win one because so far by their performance it's looking like such as they are usually under little to no pressure playing the Ligue 1 as compared to how it usually looks like playing the champions league, even their performance the difference is usually obvious.
The effort they put I to the champions league sometimes looks much more better than that which they put in the Ligue 1 because it feels like they have got some level of assurance with the Ligue 1 and their dominance in the Ligue 1 is always wey pronounced, they almost had the league title for ten years straight except that they got interrupted at a point but they have had it for nine years with little to no interruptions from other teams. It very possible they can be able to achieve similar results even in the absence of mbappe because the league is actually looking mediocre with the way teams from the league perform outside the Ligue 1.
Real Madrid will be having mabappe bringing his outstanding performance to them and complementing their striking force which has seemed to be lacking i in a very long time before his arrival, I believe it's going to give PSG a set back for awhile especially when they will be performing outside the Ligue 1 but within the Ligue 1 they may suffer the set back for only a few while after which they will most definitely get back on their feet.