Girl's got a lot of layers.
Gretel's hair is actually only kept tentacly and floating by the bandages wraps she keeps in them. Without them, it goes completely flat, and she has quite a bit of it to spare.
Few people ever get to see Gretel with her hair down, as she considers it a state of vulnerability; it reminds her of when she was a little girl and struggling just to survive in the nightmare of the Underlands.