I wondered why the world was so flat :) For the slope issue, perhaps look into `dynamic_friction` in https://docs.rs/avian3d/latest/avian3d/dynamics/rigid_body/struct.Friction.html. There's different friction values for "static" cases -- sitting still, even if you're on a moving platform) vs. when a collider is moving along a surface ("dymamic").
The art style, fonts, and music all work together quite nicely!
For gameplay, I recommend on earlier levels spawning more mice than needed for the goal. I often find myself failing to find that very last mouse...!