The default World coordinate axes are defined so that
+x points right
+y points up (gravity in -y direction)
+z points out of the screen, in three dimensions
The vertical direction or up-and-down is determined by the gravity vector direction (acceleration g) relative to the World axes. Gravity is a background property of a model that you can reset before starting a simulation, but does not dynamically change during a simulation.
See Configuring SimMechanics Models in Simulink for displaying global mechanical properties of SimMechanics models.