...And it would also be fine from the in-universe point of view (seeing as Starfleet at that time point is neo-conservative in so many ways), if not for this not really fitting the plot at all.
Janice Lester does not seem to be angry about not being able to become starship captain. She's angry solely at Kirk, for not being able to have Kirk, and subsequently (deeper into her madness) for not being able to be
Kirk. She mentions no ambition of becoming a starship captain, and no hatred of Starfleet - it's all about Kirk, Kirk, Kirk.
The captain thing is just something she has to tackle on the side, something she tells she had to spend extra time preparing for. Only Kirk, in the inquest, ever suggests that ambition would have been among Lester's motivations - and he simply guesses wrong.