[rick-l]

You need more info.

I see

1. a conduit fitting (If it goes back to the main box it provides a safety ground)
2. the box is connected to the bundle of bare copper wires
3. Old cotton covered rubber insulation which may/may not come from an armored cable

I am not sure what the code says about grounding the body of a light switch in a metal grounded box. The dimmer may also need a discrete ground for EMI reasons. It may be fine to provide the safety ground through the mounts, maybe a real off topic electrician will respond.

Breaking off the plaster ears reduces the wattage the dimmer can control???? I haven't hear of that one.