Umm..someone who can’t get a joke may also fit the dumbed-down definition of a shoiteh.
But if you change the person’s capabilities, he is no longer a shoiteh. IIRC a shoiteh is someone who doesn’t even understand that things fall down and not up. There was a man in my old shul who was born very delayed (his mother had an infection when she carried him L”A), and he was able to be taught enough to put on tefillin. A shoiteh? I don’t think so or the rav would never have told him to put on tefillin. His capacity was far lower than that of a teenager with Down’s who also davened with us at times.
But 2 generations ago at least one of these people would have been hidden away in a home and never would have learned anything. He would have been classified as a shoiteh.