She doesn't look like a mix. She could be either a Leghorn or a Plymouth Rock but since you ordered a Plymouth Rock I'm think that. Normally they aren't slim but hatcheries these days are messing up the 'norm' for breeds. What color are its earlobes?
It's a white Rock. Hatcheries don't exactly breed towards the SOP. Their objective is to produce the maximum number of chicks possible. As a result hatchery birds often only vaguely resemble the breed.