Yes, the pillow stuffing will work as acoustic stuffing as long as it has not been overly compressed.
Brand name stuffing such as "Acousta-Stuff" and similar products use fibers that have greater surface area than regular polyfill and tend to be more effective. But regular polyfill still works great and is cheap or even free. In most car subwoofer applications the stuffing is not that important so the pillow should work just fine.