Was Ryuma a dick? He seemed like a dick in Thriller Bark.
Also if Ryuma was that weak in zombie form, how strong would non-zombie Oars be? He'd kill the Yonkou and Admirals on his own.
Wasn't Ryuma way later than JoyBoy? The world collapsed in JBoys time, then was weakened and has been slowly getting stronger since then. If the world wasn't islands in JBoy's time but was continents, people would have traded techniques and technology more and fought more. Then the world submerged, technology was lost and trade and war declined, until seafaring technology was improved. Obviously this is all only speculation.