I have just measured the power consumption of a GPS mount, = 0.123 watts.
That is a tiny amount, and far less than your car alarm system, key fob receiver, clock, radio, etc. will be using.
So nothing to worry about, unless you often leave your vehicle undriven for multiple weeks at a time.
I'm a bit surprised that can melt snow, but snow is a good insulator so the heat can get trapped underneath it and build up.
It is possible that the heat is coming from the air in the car, the black plastic case of the camera may be absorbing a lot more heat than the windscreen glass alone, and is transferring it into the glass through the mount.