For people with arthritis, is it painful to pop knuckles, like hand knuckles?

Maybe, maybe not. Cracking a joint moves the parts of the joint, with the cracking noise coming from cartilage surfaces sliding, from movement of a little air-space from one place in the joint to another, or from some other non-traumatic event. One person with arthritis inflammation in a joint can feel better and looser at the joint after cracking it, while another person can feel more joint pain after cracking it.
Arthritis. It isn't painful to 'pop' knuckles. It isn't advised to do so because chronic 'cracking' of the knuckles could lead to ligament loosening of the joints predisposing to worsening of arthritic joints this causes more pain and impairment of the hands/fingers.