You're in a car, eating an apple. You finish, roll down the window, chuck the core out. It lands in the grass bank at the side of the road.
Just to set an picture of the scenario;
It's an A-road, through the country side (no pavements) and a main route so busy, and nowhere that you'd be able to stop to properly dispose of the apple core. You don't have anything to wrap up the apple core to keep hold of it until you reach your destination. Your only other option would be to hold it for the rest of your journey.
(Order of options is not an indication of my thoughts).
Is it littering to throw an apple core out of the window?
YABU - Throwing an apple out the window is littering and filthy.
YANBU - Throwing an apple core out of the window is not littering. It will decompose.