As long as you limit your other foods that are high in carbs then eating Popcorn on a Ketogenic diet is perfectly fine.
Popcorn is pretty low in calories and carbs and helps you feel full while keeping the calories and weight down.
So while you're on a Ketogenic diet or any other diet trying to lose weight the popcorn is a great healthy snack for the weight loss and Ketogenic diet.
However eating too much of the popcorn could increase your weight gain but eating a bowl or two of popcorn per day is okay when you're on a Ketogenic or other diet.
Popcorn is also packed full of nutrients as well and when eating Popcorn on the Ketogenic diet you should keep the butter off of it because the butter is fattening and can mess with your weight loss and the Ketogenic diet.
But plain popcorn is okay on a diet including the Ketogenic diet.