56% of Americans spend at least one year in the top 10%, and 73% spend one year in the top 20% [1].
[1] https://www.aei.org/carpe-diem/evidence-shows-significant-in...
American mobility isn't bad because people can't make it to the top - you can, more so than almost anywhere in the world.
It's bad because a significant portion of people at the bottom stay stuck at the bottom their whole life. Compared to countries in Europe, this is a lot worse.