The very word, Joker, naturally could have inspired card designers to depict jesters, clowns and other pranksters. Oddly, however, the Best Bowers and early Jokers show much greater diversity. We find not only the trickster types but also children, stage characters, animals, etc.
A more simple theory is possible. After the introductions of the Best Bower into Euchre, Americans equipped other card games with an extra card (usually as a wild card). Perhaps this is the stage in which the extra card became known as the Joker—meaning one that changes character or pops up unexpectedly. Designers would have tried to create some new imagery for this wild card. The choice of a jester is logical, not only because of his unpredictable behavior, but because he complements the court cards. In Europe, after all, the royal court really was home to jesters, jugglers and other entertainers.
Nevertheless, it is certain that the Joker card itself was not a European invention. It is one of America's most picturesque contributions to the history of playing cards.