Polyester batting doesn't shrink. Cotton battings shrink within a certain range depending on the brand, whether it is mixed with other fibers (Hobbs 80/20 is an example of a mixed fiber batting) and how the batting was made (needlepunched or not, needlepunched through scrim).
I use cotton batting (max shrinkage about 6%) along with fabric that is not prewashed. They shrink together to give the traditional crinkled look.
Results could be considerably different using fabric that is not prewashed with a polyester batting that does not shrink. I've never done that, so can't speak from experience, but I could see how that could be a bad combination.