My assumption is that the Kingdom was likely ruled by a D or Ds and was good, while the 20 kingdoms could have been bad or evil. But I think they were all bad, and the main reason for Blank Century if we go by the reactions of those who found out is that horrible things have happened. Chances are the government don't want anyone to know what happened because if people knew, they'd lose faith and/or rebel, causing the government and even Tenryuubito to lose influence. As Hannibal concluded, the World Government is likely using propaganda to look good even though Tenryuubito likely are the opposite.
One thing I am sure of 100% (other than the Void Century being full of horrific tragedies) is that most of the Ds somehow stood against the kingdom/s. Maybe they supported the Kingdom but due to one reason or another, lost to the 20 kingdoms but were still makin waves. If Riku and Vivi's ancestors were as kind as they were but still opposed the Kingdom or common enemy, it either indicates the enemy was bad or the other rulers manipulated things.
I think Joyboy could have been to the kingdoms or Kingdom what Luffy is to Tenryuubito, at least, or most likely, what Dragon is to the World Government. Maybe Dragon represents the will or actions of Joyboy while Luffy and/or Roger represent his personality, if not charisma. But one thing seems to be a given, Joyboy was a good person.