Mbappe is an excellent representation of the new generation of football. Much too early, much too soon, along with rapidly accelerating training methods and science meaning players are hitting their ceiling very fast.
Practically hasn't improved as a footballer since he was 20 years old. He is possibly regressed from that point.
Part of that will be the fact that having risen to prominence so quickly, PSG and France demanded star player quality out of him game in and game out, which limits focus on his development as a player. In the midst of that, it was not identified, or cared about, that Mbappe clearly lacks versatility and understanding in switching between attacking positions.
He could still turn out to be a massive hit for them, but it depends on if they will take the time to retrain him and focus on developing him as a player rather than focusing on him as a marquee signing. He's become one dimensional after all those years at PSG and needs much more tactical understanding to be able to fit in at Madrid.