I have noticed the same uptick in bot-like behaviour there. The part I struggle to square is, why so much of it is so useless?

It's maybe account laundering, but on any popular post you'll see at least half of the comments are tangential at best. They're not an expression of anything a person would express, like replying with just skull emojis to a random news post, or saying "he really said" with an exact word to word recreation of a throwaway quote from a video. No one ever replies to these posts, they get like 2 upvotes (if that), the platform doesn't reward them at all but they constantly appear in a very artificial looking way.