Lots of 'variables' there: Size of quilt, weight of quilt, fabrics used, batting used (some materials will soak up and/or hold moisture more than others), size of bathroom (will moisture diffuse easily or not), ventilation of room, how much moisture is actually created (family of 6 having steamy hot showers versus one quilter soaking in a warm bath, for example), etc........ In principal I can't see anything wrong with it. Lots of people have curtains in their bathroom, and they are ok.
I keep thinking that it would be nice to have a little wallhanging with pockets in the bathroom. It would reduce the amount of clutter on the window sill. But then again, I have never been known for being tidy......
Lorchen