The man in the striped shirt saw the voyeur looking into the bathroom. He knew the voyeur would return to spy, so when he was in the bathroom, he slowly turned his head left towards the voyeur, and offered him a leering mocking grin.
Or, when the voyeur first looked into the opening, he was alone, and he fantasized seeing the topless woman, but there was no topless woman, no one was in the bathroom. The man in stripes startled the smiling voyeur back into reality. The voyeur returns to the opening to resume his fantasy, but all he can see is the face of the man in stripes with a wide foamy leering grin mocking him.
The theme may have been - just because she has a beautiful body does not mean she has a beautiful face; her face was completely obscured, but it was symbolically replaced by the man in stripes with his wide leering foamy grin.
Be careful what you wish for, you might get her but she will not be what you thought she would be.
All of these is meaningless, Polanski was given specific instructions by his collge professor to make this film short.