You'll find this with almost all rear-mounted racks. They always recommend in the instructions that you use a trailer board if the cycles cover the plate or rear lamps. I must admit, it's not something I've bothered with as the lights can be seen clearly through the spokes on my bikes.
Thule and Mont Blanc (and probably some others) make a 'high-mount' system which holds the bikes at rear-screen level away from the lights and number plate however none list the Mini specifically.