From my understanding, it's possible for Wario, Bowser, and Morton to have good speed and good acceleration, but it does require you to get good at inside drifting. If you're only good at outside drifting, however, you're pretty much stuck with the Biddybuggy, Landship, and Mr. Scooty, and they all have a negative impact on speed.
The image below is simply an example that you can refer to.
Of course, average speed can still get the job done, as I've seen some of the heavier lightweights win online, and they can only get up to 3.75/6 speed.