Title says it all. Maddeningly, you can’t use a Magic Keyboard’s trackpad in Sidecar—at least on Catalina/iPadOS 14. The input just isn’t recognized in the Sidecar “desktop.”
If any brave soul out there is running Big Sur while owning an iPad Pro with the Magic Keyboard and happens to have the time to test this… let us know? I may be willing to switch to Big Sur if it works!