That's all it is, a rumor. She appeared for only a few seconds and an entire backstory was created by the fandom. She was a background pony thrown in as a visual gag.
That being said, my own idea is that Screwball is actually Derpy's cousin, and her favorite one as well. She acts screwy because she lives out in the Wild West (get it?), in the town of Appleloosa, where she is employed as the village idiot. And she has the business license to prove it, too.