Alt text must provide context for people who cannot see the image. Describe the image's context and purpose. Do not write alt text for cute or clever jokes based on visual content you expect users can see. This is an abuse of alt text, as it violates the whole point of alt text.

Or you can do both.

There _is_ room for being jokey or going "ah, it's [that] reference" when you've done the job of proper description, right?