> Is there anything better to gauge creativity by than perceived surprise?

I think there has to be, since such surprise can be generated through purely random mechanisms, and I don't think anyone would call a purely random mechanism "creative".

If it were purely random it would generate rubbish.

Not necessarily, but even in the cases where that's true, there will be the occasional result that surprises (in a good way).

This reminds me of when I would be surprised by AI bots I coded up for video games. Yeah, sometimes they beat me, and their strategies were often a mix of RNG and heuristics so I was certainly surprised by their behavior on many occasions. But would I consider them creative? No. Would they fool some people into thinking they had creativity and agency? Sure.