No, it wasn't. "popcorn" is the password after the reset that was posted on /b/. If you look at the screenshots, you'll see that her password was reset using the "Forgot Your ID or Password?" feature.

To reset a Yahoo! Mail password, you need the person's birthdate, zip code, and answer to their "secret question". That information is easily accessible for public figures like Palin. Try it sometime with your friend's email/screenname and Facebook; it's quite easy.