I would think that it isn't because it doesn't hold pee, the pee goes right through it to the absorbant material. So if that was the outer the clothes would get wet. You can use fleece, but I am pretty sure it is windpro. Check out the diaper making forum on here.
200 and 300 wt. fleece is good for outer as long as it's not for a heavy wetter. Windpro fleece is excellent for all outers. But you should try wool! It's even better! I know it costs more but it's one of my favorites to use for outside of my baby's diapers. HTH