It was her favorite Barbie nightgown. Priscilla White knew it, but evidently Patsy had forgotten - it was JonBenet's lucky nightie that she always made sure she wore the night before a pageant - and made sure it was packed, if going out of town for a pageant.
Patsy thought it was ratty.
Pasty certainly had a very bad memory. Didn't remember her daughter's favorite gown and didn't remember the bear which JonBenet had played with enough to redress it and put one of her earrings on it. How many other things and events did Pasty forget?