Mongols wouldn't assimilated even if they ruled 200 years.![]()
Honestly it depends on what you call assimilating. In the very least they would have changed significantly just from the Kaldun principle (that once a nomadic dynasty conquers a vast sedentary nation, keeping up that nomadic lifestyle is next to impossible over the next generations) which in general means they would appear far, far more Chinese than your average Mongolian on the steppe, even if like the Manchus they speak their native tongue at court and segregate their own populations out.