User blog comment:Doctor Rivers/Why Yang's Hair is Important to her Theory/@comment-4010415-20151114213534

I dunno if Yang grew her hair to look like her mother's, to be honest. Her quest to find her mom is all about finding out why she left, not to reunite with her. Her goal is to find her so she can ask "What could possibly be more important than your family?! What could possibly be so important that you would abandon us?!"

Lindsay has mentioned that Yang's power comes from her hair, and we've seen her hair glow and set on fire, and the tips of her hair even become semi-transparent. It's likely that her hair is where she stores the energy she has built up for her Semblance. This also explains why she was able to use her Semblance against the Ursai during the initiation even though neither of them had landed a hit on her.

If her hair is short, she can't store much energy, so her attacks can't be very powerful. She needs her hair to be long, so she has more storage space. If her hair gets cut or pulled out, that's less storage space to work with.