After all, this would mean that Janice Lester wanted to be a female captain.
And she did not.
She very explicitly wanted nothing to do with space or Starfleet, except if it involved being together with Jim Kirk. She hated her career, her "space job", and discontinued it when it turned out Kirk had blockaded himself inside "his world" which allowed for no girlfriends.
But, as I interpreted it, Janice's purpose in taking over Kirk's body was to
become a captain. And she was willing to lock him up, away from her, forever, in order to do so. She hated her career because it was
dead-end, because she would never be able to reach her highest ambition. And she had grown to hate Jim Kirk because he did not share her frustration, so he became the target of her plot. (There was an element of opportunity, of course.)
Yes, she was nuts, and wouldn't have become a captain no matter how egalitarian the Federation was. What was obnoxious about the episode, as written and presented at the time, was the implication that only a woman who was crazy would not accept the limitations of her gender.
Remember Kirk's line, "Her life could have been as rich and full
as any woman's." (Emphasis mine.) And the even-worse response (from Spock of all people!) in the Blish adaptation: "If only she had been able to accept the limitations of being a woman." Thank goodness that latter line never made it to screen, or we would have no room for interpretation at all.