Sure it's okay to give unpopped kernels of popcorn. Some of the more expensive feed has popcorn, sometimes referred to as flint corn, included. It's given to gamecocks to help get them 'on point'.
Don't give it in extremely hot weather - they'll explode! Not really.
If I'm making homemade popcorn I would go ahead and give the chickens the unpopped kernels, but because store bought prepackaged popcorn has all those artificial additives and everything I won't give it to them without washing it off first. All those chemicals do I number on our bodies, so it'll do the same to them. I know, I'm probably just paranoid, but I'd rather be safe than sorry!!!
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 pieces left over in the bottom of the bowl.
A wise man once said, "Everything in moderation." It's a good rule to live by.