I think we should delete them.

• What makes an answer a joke answer?
• How many joke answers are we ready to have?

Since it's not really possible to have a definite answer to these two questions, it will make moderation harder, and may cause undefined behaviour from passing-by users; I'm afraid every question would get jokes as answers, that these answers get up-voted 'cause they're funny' and obfuscate the more serious content, ultimately leading to a joke Q&A site.

I don't mind jokes in the comments, though.

I would use caution in allowing joke answers.

• What makes an answer a joke answer?
• How many joke answers are we ready to have?
• What if the question OP finds a joke funny enough to accept that joke question? (I'm thinking about if a duplicate question eventually pops up, would we mark the newer duplicate and point to the accepted joke answer?)

Since it's not really possible to have a definite answer to these questions, it will make moderation harder, and may cause undefined behaviour from passing-by users; I'm afraid every question would get jokes as answers, that these answers get up-voted 'cause they're funny' and obfuscate the more serious content, ultimately leading to a joke Q&A site.

I don't mind jokes in the comments, though.