PA18-150 Mixture Set too Rich?

I don’t do too many x-c flights with my Super Cub but recently climbed to 4500 MSL for a trip and the engine was running rough by the top of climb until I leaned it out. Is that typical of an O320? Or is my mixture set too rich?

A simplified explanation: That's why you have a manual mixture control. At you go higher the mixture normally gets richer because the air is thinner. Some engines show roughness sooner than others.
If your throttle was wide open when the roughness appeared it would be because a carburetor normally richens the mixture at that setting for internal engine cooling. Above that altitude it is safe to lean the mixture with the throttle wide open since your power output will be reduced by approximately 1" of manifold pressure per 1000 feet.