It really depends on the situation. I was on a Continental Express Jet IAH-DAL flight that was delayed for hours. I might have gotten there quicker had I simply walked. Our gate was changed numerous times. While boarding the plane, a passenger tried to use the lavatory. The flight attendant got on the public address system and announced "Sir, the lavatory must remain closed while boarding, taxi, takeoff, and landing." She then asked him over the address system, "Well, didn't you have enough time to use the restroom while you were in the terminal? I don't know what you were doing." I didn't find that funny one bit as we had changed gates several times without much notice. When we finally boarded the plane, the flight attendant notified us that we were now delayed because the ground crew could not find our bags to load on the plane. During the safety briefing, she had a glib attitude about her. She kept rolling her eyes as if this was nothing important...you all know how to buckle your seat belts. I appreciate humor, but not at 11:30 at night after I had been in IAH for hours after three other flight segments. I am quite sure that she knew we had been delayed for a while.
So...I like humor, but flight attendants might consider the situation of the flight as a whole.