Are you familiar with Daniel Dennett's 'Competence without comprehension' ideas? He points out for instance that even a very simple computer is far better at division and exponentiation than even the smartest human. It does this without any comprehension at all - a pocket calculator is far too simple to have any meaningful understanding of anything, but that has zero impact on its functional usefulness.
Another favourite example of his is evolution by natural selection.
Annoyingly I couldn't find a brief online article on the topic by the man himself.
Any criteria we can come up with for computers would then need to be applied to people. But how could we do that, when humans answer questions so differently? Do we ask one human? A sample of humans? All humans?
I don't think we'll find a criteria that delineates computers' "understanding" from our own. Perhaps the regularity from a sample of computers would be an indication.
All this is to say, what makes us think that humans understand subtle jokes?