Dreaming of a joke may be the subconscious part of your mind telling you that it is time to be serious. A joke is something to lighten the mood just temporarily. Thus, if your mind is dreaming of a joke, this is the temporary moment of relaxation and humor before diving into something more serious. Perhaps there is something that you have been putting off or procrastinating upon in real life; it is probably your mind knowing that you are putting it off, yet you do nothing about it. Therefore, with this knowledge, you should dive right back into your work and be serious. This dream can also be implying that you feel like you are not being taken seriously and you feel frustrated by this.