This is a question that many people may struggle with based upon the fact that Helen (from The Miracle Worker) is blind.
For some people, as seen with Helen, it is the idea that they are doing something "normal" that makes them feel normal. Given that Helen cannot see, it would be curious (to some) that she is standing in front of a mirror and "looking" at herself. What this action is alluding to is the fact that she can "see" herself in her own mind.
The behavior is one that is simply typical of a girl who wishes to see what she looks like. The fact that Helen is blind has no impact on her; instead, it is the fact that she can imagine what she looks like in her reflection of the mirror that has the greatest impact on her, not the reflection itself.
For example, it would be no different if one were to imagine them self sitting on a beach on vacation as an imagined, or envisioned, getaway. Here, Helen is simply imagining what she looks like. The trip to the mirror is symbolic. The act of modeling the clothes is not symbolic though. One does not need to see their reflection to show off clothing.