Background Pony #9563
She says “afrown” not “of frown”.
It’s attaching the prefix “a-” in the second sense explained here: https://www.britannica.com/dictionary/eb/qa/Words-with-the-Prefix-A-
So “face afrown” means “a face in the state of frowning”.
If you looked up the lyrics on google, sure enough they’re wrong.