I've never seen the need or use for a mirror while camping. However, if there's ever been a time when I need to check something out (look for ticks on my back, or see how badly I just cut up my face in a fall) I've taken a photo of myself with my digital camera, and then zoomed and panned in the screen on the camera to see how things look.

That said, I'd think that as long as you pick your outfits ahead of time, you shouldn't need to use a mirror at the music festival.

Andrew.