MDeGe- Welcome to Autopia. Four possible problems with diapers come to mind:
1) They might be cotton/poly blends that will mar paint in and of themselves.
2) Even if 100% cotton, they might be too coarse, again, marring the paint through abrasion with the cloth itself.
3) The stitching might cause marring.
4) Diaper cloth has no appreciable nap, so there's nowhere for dried product (or contamination) to go, so stuff like that gets ground into the paint.
Diapers used to be "the thing", but I've never found any that I'd use on MY paint. Since I have them, and they wear like iron, I use them for other stuff, but not on paint. These days you can do a LOT better. Even many 100% cotton towels would be a better choice, but as you can tell MF is the hot ticket now.