Visualise if you will, the AFTERMATH of an aircrash. Imagine the result of a smarmy, 'clever', 'funny' SAFETY briefing, with passengers still trapped in the fuselage because they did not hear the important message of how to exit; or the floating bodies who did not know how to don or inflate their life jackets. Consider how the investigators will examine the recording of the SAFETY briefing and where the fault will be sheeted home.
I like to develop a good rapport with my pax and humour is a great tool, IN ITS' PLACE - which is NOT the SAFETY briefing.