Yes, this diagram needs to be described - that's really a much more important point then the joke. That being said, the joke is not strictly speaking entirely decorative. If the whole document were presented as a book, the visual joke wouldn't work, but there would be some other joke to lighten things up.

Ultimately, alt text and descriptions should make the content functionally equivalent. Losing the description of the joke creates a light experience for a sighted reader anda dry one for a blind reader. Not equivalent.

that said, the web is ridiculously verbose already. A default NVDA configuration can overwhelm a newly visually-impaired person with nested graphics in lists in tables in sections on the web. Speaking as someone who appreciates information density, decorative images to me are useless. The BBC is a great example, one of their "long-read"news articles last month had 5 "decorative line" images used as visual splitters within the news article. I was already navigating by paragraph; I just didn't need that.

The problem is, if you try to please everyone by making them optional and turn them on by default, it's effort to turn them off and vice versa, not to mention the cascading effort required by web developers, content creators and CMS engineers to implement the feature in the first place. I imagine half the blind people will say "yes, we need a literal, detailed description of ev-ery-thing we can't see so we're treated the same as the sighted", and the rest will say "it's cool, give us the important shit and leave it at that". So the scales of opinion will hover somewhere in balance. Nothing will change. About half the world will go one way and half the other, as things are pretty much now, and things will just hold in a silly sort of arms race of homeostasis until something big happens to shake things up.

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I would argue that there could be two separate pieces of alt text here - the diagram and information on that and the Squirrel so the joke is not left just for sighted people to enjoy.