You're just looking in the wrong place. Her "nowhammy" trait is given when Xavier catches you doing naughty things with her in public (in the script_Scenes file):
By default, she acts as if she has "whammy" active, which refers to her using her powers to mindwipe people who see her exposing herself or otherwise doing things she might be embarrassed by. Once Xavier forces her to stop however,, she gains the trait "nowhammy" which can be toggled with "whammy" by either of two new dialogue options that only are available if she has either of these two traits. One dialogue option is available when you perform a plan on Xavier, allowing you to make him remove or reinstate the ban on her using her powers that way. The other is in Jean's Dirty Talk options, but she'll refuse to swap from "nowhammy" to "whammy" via this dialogue option if you haven't had her perform Plan Alpha against Xavier yet, out of concern for how she might be punished for going against the ban (even if you had him remove the ban, since the game doesn't actually track that separately from her agreeing not to do it at your request).
Anyway, you need her to have the "nowhammy" trait for her to gain "exhibitionist", because without it she's automatically removing everyone's memories of seeing her doing anything beyond kissing in public. At least that's the lore justification.
