Yes, Jackson was likely mentally disturbed and should have realized that relentlessly harassing another celebrity was unbalanced behavior. Prank calls that never cease are no better than stalking, and Jackson made a nuisance of himself. He was unable to recognize another person's boundaries and behaved inappropriately with no respect for Crowe's privacy.
Michael Jackson was severely mentally disturbed. This is clear by his behavior, including relentlessly placing prank calls on Russell Crowe. He also engaged in other behavior, such as bringing young children to his bed and holding his own son dangling outside a window. He was raised by an abusive father, which led to his mental problems.
Mental disturbance is not exactly linked to prank phone calling people. If they were friends, then it does not seem so odd. It is wrong to classify someone as mentally disturbed based off of such an inane activity and the people probably don't even know all the details of the story because we were not there.
No, Michael Jackson was not severely disturbed. He suffered from the chronic condition of being a celebrity. He quite literally grew up in the public eye, and never really had an actual childhood. Was he socially immature? Most likely. There were many things that he was not able to do as a child that most everyday citizens take for granted, like riding a bike in the park or meeting friends for a movie. He was immensely talented, and extremely eccentric as well as socially awkward. So yes, he was painfully weird, but that doesn't mean he was mentally disturbed.