Yes, it does. I don't use it often, but I did test it and it seems to be fine. MotionVFX has publicly stated all of their products work just fine on High Sierra and FCPX 10.4. If you have any issues, contact them directly, they're tech support is really good. You can contact them directly via support (at) motionvfx.com.
BUT, Simon did announce via Twitter that they have an mObject 2 plugin coming. It is in development, and has great promise. I'm very excited to see what they deliver on that.
Side note; if MotionVFX can bring all this 3D object file use into Motion, why can't the Motion development team just build that in to the app itself?