HTML and CSS are declarative languages and still arbitrary source
The demarcation criteria is a bit tricky, one could argue that HTML and CSS can't be executed, but it's probably not a debate worth having. Can you think of a better way to describe this concept?
But you have to see that even a "federated" social network like Mastodon has better quality user content than Twitter, even if it has a fraction of the content overall.
I have a rather superficial knowledge of both, but for the sake of argument I'd be willing to accept the premise ignoring all the furrys etc. From this follows the question is the barrier to entry to Mastadon what improves the relative quality over Twitter, even if the quality is still poor? A natural direction of thought would be from this to ask whether Mastadon is primarily distinguished from Twitter in form by its barrier to entry. To gain insight into this one could ask why users switch to Mastadon from Twitter.
Is there such a thing as "herd content"?
I think what you might be referring to here is “The Spectacle”, although the scope of what you're talking about is more limited than it.