You can't expect a player to perform well straight away in his first season and win everything with the team. You have to give him time to adapt and create a bond with his teammates, who are all very strong, and so it's also a problem for him to find space within the team. I understand he's strong, but it takes time.
To be fair, Mbappe played amazingly, in all competitions he scored nearly 40 goals if I am not wrong, I do not have the exact number but he was said to have the most goals scored in a debut season. This is why I believe the issue wasn't really him. The issue is that whenever Mbappe plays for a team, the focus shifts to him, and that causes others to not play that much better.
I have always said the exact same thing for Ronaldo as well, the team plays so he can score and they win that way, and not a team football like how Pep makes the players play, even Ancelotti didn't focus just on Vini for example. This is why I believe we should see them change the way they play, and Xabi is trying that, they are trying to make the whole team play together, and not focus on a single player.