This subject seems to come up frequently.
Passenger addresses seem to elicit a lot of myth and folklore stories.
The truth of the matter is that they are part of the professional communication from the flight deck to the cabin. There is no question that a bit of humour does have its place in certain situations and usually when it happens it is spontaneous rather than planned. However for the most part the announcements are done in a manner that conveys the same professional attitude the speaker has to his or her other tasks, and for the most part that it what the customers want to hear.
Humour is a very subjective thing, and what one recipient may find amusing another may definetaly not. A passenger who has been subject to any number of the stresses induced my modern air travel, may well be less than entertained when the Pilot decides to portray himself as a comedian or clown, and it is not something that is often encountered.
As with many things, it is a question of common sense and awareness. People visiting a comedy club expect to be entertained. Passengers on an airplane expect a high degree of professionalism and since their only contact with the flight deck crew is over the PA, this needs to be handled carefully.
There are many do's and don'ts, but these vary from individual to individual and from situation to situation bearing in mind the overall requirements.