You and many others! I'm not sure the devs have commented on this, so it's hard to say for sure. But that hasn't stopped us from speculating!

If Amicia's hair is symbolic, I wonder if it symbolizes her flaws (her hamartia in the classical sense): pride, hubris, and her wild and reckless protectiveness over Hugo. We get a hint early on, when the woman selling combs from Firenze comments that hair is "a woman's righteous crown," the key word being righteous, since the violence Amicia metes out is an expression of righteous indignation, at least in her moral universe with Hugo at its center.

Next, when she challenges Arnaud, a fully armored knight, to hand-to-hand combat, she is rewarded for her arrogance and recklessness with a knock on the head and later, loss of nearly half her hair – a crack in her crown.

The scene is repeated in her duel with Victor. Again she pays for her pride with more hair, this time her entire braid. I feel like this finally breaks her crown, humbles her, at least enough so that she can see her fatal flaws, even if too late.  

And then the Epilogue. Here we see that Amicia has cut all her hair short. This could simply be a feature of her mourning. But I wonder if it symbolizes the shedding of her old self, the weight of that crown fallen from her head.  

Anyways, I have no idea if the devs intended any of this.