I found it interesting because it reminded me of many of the tribal masks worn by native peoples he photographed in the 50s and 60s. I simply liked the texture of the mask (looks almost like burlap) and the smoothness of the "mud" that Penn probably added to define a mouth and nose. Its that wonderful texture and hollow eye sockets (and whispy bits of hair?) that makes it interesting.
If you look at his work with Aboriginal peoples, masks and such adornments were considered beautiful and a sign of status and position in the group.
Here we see the mask as an ugly artifact that somehow will metamorphasize the person underneath. This mask is designed to facilitate covering up something (self percieved ugliness?). Aboriginal masks project an inner personna of the person wearing it. Two different cultures and two different ways to look at masks, hiding something or projecting something.