It sounds like your problem isn't with funny images, but tonal mismatch. In that sense I agree with you - if the article's tone is lighthearted, use lighthearted images. If not, then don't.

I would expect the "a monad is just a monoid in the category of endofunctors, what's the problem" article to include humorous images. I would expect a serious tutorial about monads to not do so.