It’s not littering. And it’s not OK to do that because it’s biodegradable, it’s OK because crust will be eaten by birds/rodents/insects in about a day. Throwing away a paper cup because its biodegradable is, however, wrong.
If Leviticus was written now it would go something like that ^ “Thou shall not litter, for it is unclean. Excepting biodegradable items, for they shalt be fed upon by the beasts of the field. Paper cups are unclean and not cool dude. Any man who lays a paper cup upon the ground shall be sent into the desert for 3 days and 3 nights to do community service”