What does the symbolism of the wallpaper represent about women being trapped by society's norms?

I think you've already identified in your question what the wallpaper represents. A key symbol in the story, the yellow wallpaper is interpreted as a symbol even by the narrator—she is sure it is telling her something, but she cannot figure out what. As time goes on, the symbolism of the wallpaper becomes clearer to her: there is a woman trapped within it, seemingly attempting to escape from behind bars. She is like an animal in a zoo; she represents not only the narrator, but all women.

From context, it is clear that the narrator feels trapped by her own situation. She knows what she personally wants—she thinks it would make her feel better, in terms of her health, if she were allowed to keep busy and do things. However, the men in her life have argued against this and have instead forced her to do the exact opposite: "rest" in a quiet place, far away from any stimulation. This is a situation that is suffocating for the narrator. Like the woman behind the wallpaper, she feels caged; social norms and misguided ideas about women's health have hampered her ability to live her life as she wishes to.

The wallpaper can symbolize the way women become a part of entrapping societal norms and lose themselves in the process. At first the narrator hates the paper and wishes to be anywhere except that room. She then slowly becomes a part of the paper, morphing into a "creeping" woman. In order to break away from the paper, she needs to tear it down, but that still leaves her as a "creeping" woman who has lost her mind.
The wallpaper also represents how pervasive and subtle these societal norms are. They are so subtle that one can only see them when one is looking at society in a particularly critical way. Otherwise, the pattern of sexism is unseen and ignored:

This wallpaper has a kind of subpattern in a different shade, a particularly irritating one, for you can only see it in certain lights, and not clearly then.

It is only in a very specific lighting that the narrator can actually see the figure in the wallpaper.