I have got 120Hz built in laptop display. I'm using updated Pop!_OS GNOME. My external mouse is fluid as it should be. I see my mouse moving is fluid that up to 120FPS. I tested with
journalctl -f | grep FPS.
But the touchpad is not fluid as it is in Windows. While moving window I can see touchpad is not much than average 60FPS. Also I tested so my observation is true its almost not higher than 60FPS and laggy.