From what i understand the idea is to have the roller change rpm in a manner that most closely mimics the acceleration of the vehicle so the engine is loaded the same way it would be during acceleration.
In other words, the mass of the roller is going to be sized for the vehicle its meant to test, there are certain assumptions being made by the software, and those assumptions are only valid if the roller is being accelerated similar to the way that the vehicle would accelerate. If 300 hp gets your car from 15 mph to 100 mph in 10 seconds, you will want the roller to speed up at the same approximate rate.
Since your bike is variated, and the roller will be way too heavy, those assumptions will just be slightly off and you won't be getting a 'true' hp number, but you can still use the relativistic HP for tuning purposes.
The other thing about the dynojet, is that once its up to speed it can just keep going unless it has a load brake. You could get your moped up to 100 mph indicated, but your rod would likely come hanging out. The operator will probably tell you this, but you really don't want to pull much past the RPM /speed you see on the street. Its meaningless numbers at that point and your risk going home with your connecting rod hanging out the bottom of your engine.