TikTok reading it every couple seconds is definitely excessive. And frankly our tools fail us by not at least revealing that it's taking place -- new clipboard notification aside, as shown in TFA. How do I know how common vs weird this behavior actually is?

I'd only refine your post to say that it's common for apps to read the clipboard without you pasting. Right click Chrome's omnibar and it will show "Paste and go to <clipboard contents>", my bittorrent client and RSS clients prepopulate the new torrent/feed form if I have a URL in my clipboard.

Is the tiny convenience worth the ability to snoop? I don't think so. Or rather, I would like to decide that for myself.

Malice or just stupidity? I can imagine a dozen different reasons a program might access the clipboard in a loop, all of which reduce to "we are bad programmers".

Not all of them reduce to bad programmers. It could be either of malice or stupidity. In this age when data is valuable, it is better to be safe and assume malice.