How can I fill gaps in wall plates on an uneven backsplash?
There are insulating gaskets for switches and outlets like these



They would help seal against air leaks, but they might not visually fill the gaps at the edge of a switch plate.

There are also deep beveled switch plates, such as those found here. You could custom grind these down with a dremel-type tool to fit the highs and lows of the surface, but that would be a pretty time consuming task.

While you probably could caulk the gaps, you should be careful to avoid getting any into the box and wiring itself. Also, caulking will make removal of the cover plate a bit more difficult.