Humour over ACARS is something you need to be careful with. If the person on the other side is not someone you know, you might end up in a meeting with HR or your manager. ACARS messages are viewable by a wide range of people and are part of every accident investigation. Not everyone is going to get your jokes and it could come across as lacking in professionalism.
As dispatchers, workload is always something that we want addressed and kept at a manageable level. If you are send too many non pertinent messages over ACARS, you dont look very busy. It sends a message to pilots that we are not very busy. When you are having multiple flights diverting, the last thing you want are more messages piling on top from crew just wanting to chat.
Pilots have and probably will again in the future report dispatchers to the Feds for not responding to non-pertinent messages when busy. Pilots overall do not understand how many flights dispatchers are dealing with or how busy it can get on a dispatch desk.
I was jumpseating back to work one time and the dispatcher sent quite a few messages trying to be funny. The crews response to me was that if dispatchers have that much free time then we should all be sending messages like this.