You can wash the quilt again and then stretch it out a bit with your hands. You can then put it on the ironing board and steam it a bit, especially in the unquilted borders. Then lay it flat to dry.
Since the problem stems from large areas unquilted in the borders, I'd do more quilting there once the quilt dries.
To me it's really not a problem, I'm just offering these suggestions because the way it looks seems to really bother you. It's a cute, cuddly quilt.