It is true that, given their weight, cars are much better on the highway with lower wind resistance. However, motorcycles use much less fuel in stop-and-go type situations then a purely petrol based car.
Motorcycles have the benefit of lower weight and the added expense of greater wind resistance. There is not much room to improve their mileage. Of course, this assumes you are looking at a model that is designed for efficiency and not the motorcycle equivalent of a mustang.