Offred doesn’t feel sexy, so you probably should try to feel sexy when playing a version of her on Halloween.
But the costume probably shouldn’t veer so far off of the course of the original character so as to reverse — or even mock — the point of he person in pop culture being represented by the Halloween costume.
There are numerous ways to empower females, and a lot of pop culture characters who can be turned into sexier versions of themselves if someone wants to pretend to be that side of their personality on Halloween night.