I've never seen Kylian Mbappe perform disappointingly, maybe he doesn't score goals or assists or statistically, he doesn't play like he usually does, but surely many people will agree that he is a fairly stable player, which is why Real Madrid is trying to ensure he plays at a big club.
Mbappe is not God and he is not able to bring 3 points to his club in every match. He is not able to score goal in all matches and I am sure we can find disappointed matches and individual performances of Mbappe through his career so far from Monaco to PSG and French national team.
In fact, because of the presence of the captain of the French national team, many have predicted that Real Madrid will not only be able to defend the Laliga title but also the Champions League title and that is certainly not impossible for Real Madrid because they are one of the clubs that can achieve this while other big clubs are only occasionally unable to defend it in the following season, but still predicting that Real Madrid will defend the Champions League title is too early because the group draw has not yet been held.
He is one of best attacking players recent years and he will help Real Madrid to attack better next season. With competitiveness of Real Madrid, Mbappe will play for a club which usually is considered as one of best competitors for Champions League title nearly every season. It will be biggest surprise if Mbappe won't win any Champions League with Real Madrid in coming years and if it happens, we can say it is like destiny of Mbappe.
It's not easy to happen because Mbappe has a long career ahead and he will have enough time and seasons to play for Real Madrid and win Champions League with them.
Mbappé is still young, with a long career ahead of him. So if he performs aggressively from now then he can perform better later. Mbappe can give Real Madrid a good dream because until now the Real Madrid team has been strong enough, then the Real Madrid team has become even stronger due to the addition of Mbappe to the club. And these titles from the present to the future must be won.