It is a common device to evoke laughter by placing a watermelon in an incongruous situation. This is extremely easy, for just about any scene with a watermelon appears a bit incongruous. Tough for the watermelons, perhaps, but fortuitous for the cartoonist and joke writer. Among many subjects interviewed, opinions as to the delectability of specific fruits varied widely, but did not affect their amusement potential. On the whole, people get a kick out of bananas whether they eat them or not.
Wax fruits are notoriously big laugh-getters.