Popcorn in and of itself is not "bad" as a treat. HOWEVER, the extra ingredients IS. You'll want to check the ingredients. Pre-made popcorn of any kind tends to have a lot of salt and while the amount may not alarm us, we have to remember that birds have MUCH smaller bodies and systems than we do.
I do know one thing, I would never share microwave popcorn with a bird, even a little bit, cause of the lining.
While I don't think once or twice would kill them, you have to look at it long term. IF you share some now and its something you eat routinely in front of them, they are going to want some each time you want it and it really wouldn't be fair if you don't share it every time after you've shared it before. Thats why I only share 'snacks' with Skittles that I either have only on a rare occasion OR are okay for him in moderation.
Its also not just the food item, but the amount. ANY snack food can become overkill even if its only on a rare occasion. If I am having pizza, SKittles will want some crust, no doubt. I usually eat pizza AFTER he goes to bed for this reason. BUT, sometimes I eat it before just so he can have some crust. But I have it set to a routine. He gets a little bit of crust and then I give him his regular 'bird' treats and he's become accustomed to that. He gets a little 'naughty' snack and then his regular snack while I eat mine.