I agree with Shelly that getting a tuck in a pieced backing is very easy to do.
In some instances almost impossible not to. I know, you sew two fabrics together and they should be flat, but like Shelly said, unless you are using an even feed foot the bottom piece will be sucked in faster than the top piece, resulting in one being very slightly gathered. Spread over a long seam this can result in extra fabric that has to go somewhere. Have you ever cut off a flared skirt or pants leg and when rehemming had that little bloop of fabric on the top side with nowhere to go? The quilting overall is very nice. It would have been nice if as soon as she saw what was happening she called you
to give you a chance to take it home to fix the backing. But maybe she did not notice or maybe she did the best she could under the circumstances. Please don't bad mouth her to other potenial customers until you give her a chance to
do something to help you feel better about this quilt. It is still a lovely quilt and I would like to see a pic of the entire quilt front and back. If sewn down with a snug ladder stitch the pleat will be less noticable.