I've seen some light hearted pitch decks before, and as someone who appreciates comedy, I enjoy that... but I often wonder what an analytical minded or more money motivated VC/investor thinks when they see rap lyrics or cheeky humor in pitch decks. I know this is an extremely subjective question, so I'm hoping to get some input from some investors/vcs/angels rather than entrepreneurs.
Instead of calling it "comedy", aim for humor. Humor that supports a key discovery helps to anchor that insight in the investor's mind.
I've coached many pitches, and find that when presenters are nervous they try to go for an improvised laugh, thinking it will cut the tension. However, in most situations, it comes off as just that: improvised and cheesy. Which confims to the audience that the presenter is nervous and unsure.
Humor an advanced communication skill which requires strategy, planning and practice. The best humor is subtle and smart. The goal is not to get a laugh, but to get a knowing smile.