The law says that you must be able to fully see out of every window in your vehicle for safety reasons.
So yes in most places it's illegal to drive with the snow on your back window of your vehicle because it obstructs your view.
So before pulling out of your driveway or any other place make sure the snow is removed from all of your windows and that includes the back window.
If you do get stopped the police will likely just give you a warning about the snow on your back window and have you clear the snow off before letting you go.
Then if stopped again for the same infraction they will likely give you a hefty fine so always remove the snow from every window on your vehicle.
It's really the safest thing to do anyway whether it's illegal or not.