It depends on how the fall happens. Most falls aren’t funny at all to me and they’re more likely to make me cringe and go, “OOoooooo!” in sympathy. (and maybe I’ll run over and help the person up if they need it, etc.)
However, humor approximately always involves the element of surprise and often vice-versa.
So if someone does a particularly agile watusi on the way down, or launches an armload of books into the air, or takes out an entire stacked-can display at the supermarket, or miraculously tosses the baby into the arms of a passing stranger a moment before impact, that could get me to laugh even if the crash is painful.
On that score I’m with @hearkat I think it can be reflexive.