Yes, I'm confident it's wrong because it re-use a word in a way that will confuse. It implies it'll be used without a GUI and the first thought is not to think "oh, they must mean it doesn't come with its own GUI". Lots of others things that have an API do not get called headless - presumably as they're aware of the existing use of headless and thus don't want to confuse people.

Maybe you could explain it's a backend for an editor.

Or maybe "unstyled"?