"an avatar" would also cover vtubers so png-tuber is actually more accurate here
I prefer minimalist stop motion avatar to "PNG-Tuber" which sounds more like a slang description of the person that relias on said avatar variant.
Also avatars in early games were exactly that simple, so that name was here before.
Pretty sure that's the intent, since it's a play on the name "vtuber", which is used as you describe.
and if you're introducing a niche term to a broader audience who might lack that context, in an asynchronous or static medium like an HN post or repo README, providing a definition that describes it in more widely used terms is a low-effort way to help unfamiliar people learn about it without forcing them to ask
As is complaining and "flagging" when someone calls it out.