Technically yes, due to the non-linear drag from air resistance there's slightly more energy required to travel at varying speeds rather than maintaining the same average speed but without any variation. But in practice while bicycling this effect is insignificant. Let's say you have someone on a light bike who averages 20 mph but does so by speeding up to 21 mph during part of the pedal stroke and slowing down to 19 mph at another portion of the stroke. If we simplify the problem and assume he spends half the time at 21 and half at 19, then the power he puts out could propel him at 20.05 mph if he used it to maintain a perfectly uniform speed. Actually the benefit would be even less since the speed would vary roughly like a sine wave rather than abruptly switching between the two speeds.

Now putting extra weight in the rim will help to even out the speed since it acts as a flywheel, but it'll also add some rolling resistance. And since the potential gain from maintaining a uniform speed is so small (less than 0.05 mph in this example) it doesn't make sense to add the flywheel - even on a totally flat course.