I don't think it's a bad idea. I give mine stale chips when I find them in the back of the cupboard. Like I said, everything in moderation. A little leftover movie popcorn should be fine. Buckets and buckets of it, probably not.
My husband loves microwave popcorn, Extra Butter. The unpopped kernels as well as the riff-raff at the bottom of the bowl get put into the "chook treats" tub, then tossed to the chickens. None, and I mean NONE of my 26 birds, will touch the unpopped kernels. But they do like the popped bits and...