I just finished making a quilt for my GD and her American Girl doll for Valentine's Day. Used a leftover piece of poly batting; not sure of the loft, but it was in the medium range. I wanted the appliqued hearts to stand out when I quilted around them. And I like the feel of a quilt this size with poly batting rather than the thinner stuff like W&N or flannel.
I think WM poly batting would be fine for this purpose. Good luck!