Ball is probably practical. The head is probably CG.
They never cease to amaze me with what they’re able to come up with, you know? I said, ‘How are you ever gonna top R2-D2, the most adorable droid in movie history?’ And then they have this new one,” Hamill said. “I can’t even tell you his name, but you saw it in the trailer. And when they were demonstrating how they did this thing, live on set — because it’s not CGI, that’s a live prop — I was just amazed.
He wouldn't have said this if it was half-CGI. Especially when he is so amazed with the tech behind it. A remote controlled rolling ball is not impressive, at all. Mark Hamill's description makes no sense if the head is CGI.