The fact that someone is “only joking” is not a valid excuse for speaking loshon hora.
One Purim a group of Radiner Yeshiva students performed a skit in the house of the Chofetz Chayim of sacred memory. Among other things, they jokingly alluded to an episode that had caused a large controversy in yeshiva circles. Immediately, the Chofetz Chayim admonished them softly, “Even on Purim one must refrain from loshon hora. ~ Guard Your Tongue