The Cambridge dictionary defines practical joke as
a joke that makes someone seem silly and involves a physical action rather than words
As @FumbleFingers commented "practical" alludes to physically doing things, putting them into practice (as opposed to ideas, words, verbal jokes)
This is confirmed by Wikipedia
A practical joke is "practical" because it consists of someone doing something that is physical, in contrast to a verbal or written joke. For example, the joker who is setting up and conducting the practical joke might hang a bucket of water above a doorway and rig the bucket using pulleys so when the door opens the bucket dumps the water. The joker would then wait for the victim to walk through the doorway and be drenched by the bucket of water. Objects can feature in practical jokes, like fake vomit, chewing-gum bugs, exploding cigars, stink bombs, costumes, whoopee cushions, clear tape, and Chinese finger traps. A practical joke can be as long as a person desires. It does not have to be short-lived
In this case "practical" is being used as an adjective because it describes the type of joke.