Popcorn sits in the maybe category. It is not automatically dangerous, but details matter.
Quick call
Usually yes, with caveats. Butter, salt, oil, and unpopped kernels are what cause problems.
What matters most
Only plain, air-popped popcorn with no toppings.
When to call your vet
If your dog has vomiting, diarrhea, unusual fatigue, or belly pain after eating popcorn, call your vet the same day.
Source check: AKC recommends plain air-popped popcorn only, and no kernels. Source