Yes, I have used mattress pads instead of batting in the charity quilts I make. I had not one problem with using them and it really saves me some $$$.
Does anyone remember sheet blankets? We had sheet blankets on our bed in the summer in NY. I would love to find them again as they would be perfect for the batting for a quilt staying in AZ. They are probably a thing of the past like so many other things we remember from long ago. Sigh!