You can increase your Sims comedy level by several methods like watching the comedy channel on a TV, reading a comedy skill book, telling jokes etc to other Sims, writing jokes and routines etc on a computer and so on.
These seemed to take a while so the best way I found was to buy the microphone in the skill section of the furniture store.
Once bought and placed select the “Tell jokes” option with the Sim of your choice. They will then tell jokes like their life depends on it until you tell them otherwise or sometimes when one of their needs gets very low.