It is not ethical. Hinata Hyuuga is a character, and creating fanfiction that involves abuse goes against the spirit of respecting the character and can also be disturbing to other fans who love the character for her positive traits.
Well, in general, it's unethical. Fanfiction is a form of creative expression, but when it comes to themes like abusing a character like Hinata Hyuuga, it crosses a line. It can have a negative impact on the perception of the character and the overall fan experience. It may also be against the values that the original work tries to convey. For example, in the Naruto series, Hinata represents determination and kindness, and creating such abusive fanfiction is disrespectful to the character's essence.