I don't think it is acceptable. Fanfiction is often a creative outlet for fans to expand on existing universes or characters in positive ways. Brainwashing a girl in fanfiction goes against the principles of respect and equality. It can also be triggering for some readers who may have experienced or are sensitive to issues related to coercion and control.
No. Brainwashing implies the violation of a person's free will, and when it's specifically targeted at a girl in fanfiction, it can be seen as a form of exploitation or objectification. Fanfiction should generally promote positive values and healthy relationships, not such disturbing concepts.