Finally! Someone on Reddit helped me remember the term after trying to figure it out for a few years. I'd tried the term pleated, crumpled, ruffled, gathered, bunched, even silly terms like corrugated but never could find anything close to the tube top I had when I was little.

I've been looking for undies made out of this stuff for years! Now that I know the term, I'm one step closer to my goal. Unfortunately, googling 'popcorn undies' turns up the most silly things instead of what I want. Still, one step closer though!

I will find them.

-edit-
This is a popcorn blouse, for the confused!