That's a tough one to pin down because there's usually a lot involved, house wise. Usually, it's a combo of Mercury and it's aspects and house placements.
George Carlin, for example, had Merc in Taurus in 10 with these aspects:
Sun conjunction Mercury (his speech supports his image)
Mercury trine Neptune (creativity)
Mercury square Ascendant (speech makes him look like a bad guy)
Mercury semi-sextile Venus (speech is appealing to others and gives him money)
Mercury conjunction Uranus (speech is shocking and unexpected)
Mercury trine Jupiter (speech gives him good luck and expansion and a wider audience)
You could see how all of these apply.
Then there's Jim Carrey:
Mercury 14°44' Aquarius, in House III
Mercury conjunction Jupiter
Mercury square Ascendant
Moon trine Mercury
Mercury square Neptune
You're seeing aspects to Neptune and jupiter there again.
looking at Bob Hope's chart you can find:
Mercury 14°27' ? Gemini, in House VIII
Mercury conjunction Pluto
Sun conjunction Mercury
Moon semi-sextile Mercury
Mercury trine Saturn
Mercury square Jupiter '
Mercury trine Ascendant
Mercury opposite Uranus
This time we're missing the Neptune, but Jupiter is involved again.
I would have to research more, but Jupiter seems to be a key planet in humor, at least in the charts of famous people who make their money off humor.