Rick Rolling the meaning is pranking someone.

Badger Badger Badger it's the humor of the song and animation, and the unexpectedness of the mushroom, plus some absurdism of course.

Nyan Cat is the closest, in that it's just absurdist, but also it had a catchy song and was art.

6-7 is pure absurdism. There isn't a meaning to it, and it's lack of meaning is the joke. It's meaningless is so absolute that the brain defaults to base pattern recognition to give it any semblance of meaning.

And I'm not saying that's bad. I'm just stating that it's literally a meaningless joke. Some may ascribe it's rise to prevalence as part of modernized brain rot culture, itself an extremist version of absurdist humor that began to rise in the 80s with Gen X and exploded with millennial, but evolved through very compressed short form content.

To me, I think it's maybe a prelude to an increase in dark humor again (and indeed we saw a rise for a moment in which 6-7 was given a dark twist with the mixing of SCP fiction with it's focus on voidness, emphasizing the insanity of the meaninglessness of the meme, but also the emptiness of everything) similar to millennials, since I think Gen Alpha already sees how bad the situation is for their generation even at a young age and will eventually perhaps evolve to a mixture of dark nihilistic violent humor; the meaningless of 6-7 it's expression of nihilism in a humorous way.

But that's just a theory. A meme theory.