Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
There is nothing wrong if the imam uses humor during the Friday sermon.
Answering your question, Dar Al-Ifta Al-Misriyyah, states:
It is permissible during the Friday sermon (khutbah) for the Imam to use humor as a way of delivering a certain message in a light fashion that does not invalidate the Friday prayer in any way.
It is also allowed for the audience listening to the sermon to laugh at the Imam’s humor and their prayer is valid as there is no prohibition against using humorous language to deliver an intended meaning.
Almighty Allah knows best.
Source: http://www.dar-alifta.org/Foreign/ViewFatwa.aspx?ID=10440