In this article you'll learn more about velocity scaling options in MainStage.
Unfortunately, there is no way to adjust velocity scaling globally. There’s a “more/less” slider when you select the keyboard in Layout Mode, but that doesn’t precisely adjust.
If you wanted to handle this within MainStage, you could do it per Channel Strip, in “Midi Input” tab, by adjusting Velocity scaling:
If you wanted to do it globally, you could look into a third party App like Bome MIDI translator, we believe that would let you adjust velocity scaling, then pump the output of Bome into MainStage as the Midi Input. You would want to reach out to Bome to confirm that’s feasible.