I completely disagree
She was not mentally ill. She was filled with completely understandable rage at having to be pure/pious/having her fiancée sent away/living under an unbearable bullying misogynistic father
NOT mentally ill, understandable fury.
A world with no suffrage, no power, whose only worth was pretending to be moral.
The genuine moments of feeling in the film were her deep envy at who Rose got to be, free. And her friendship with her, where she got to be a tiny bit unguarded.
What IS hideously sexist and of its time is the bit not portrayed in the film is that she was committed to an asylum because of course she HAD to be unwell
for the rest of her life 
Was she fuck, it's how a normal person would react when they had no power
Silly little letters, full of swear words she wanted to say but couldn't
A brilliant portrayal of someone not unwell and a finely nuanced performance.