@OpenlyGayExOlympicFencer
Lizzie is pretty obviously meant to be bright, whatever the defects in her education.
Oh, she's certainly bright, and a witty, sharp conversationalist, but as a pp said, she in no way approaches Darcy's highly programmatic idea of a truly accomplished woman (well, Miss Bingley's, but he agrees) who needs 'a thorough knowledge of music, singing, drawing, dancing, and the modern languages', ' possess a certain something in her air and manner of walking, the tone of her voice, her address and expressions.'
“All this she must possess,” added Darcy, “and to all this she must yet add something more substantial, in the improvement of her mind by extensive reading."
Of course Austen is mocking 'bluestockings' by making Mary, the supposedly cleverest and the most musicianly of the girls, a pedantic bore who hasn't an original thought in her head and who never knows when to stop if she's allowed to play in public.
Presumably poor Mary, knowing she can't compete on looks with her sisters, has decided to go in for accomplishments without any particular intelligence or taste for music. Austen implies as much when, at the end of the novel, she's the only daughter left at home, which forces her away from her studies and out into company with her mother, and as she's no longer being unfavourably compared to her sisters in appearance, her father thinks she secretly prefers it.
(Interesting that Kitty 'spends the chief of her time with her two elder sisters' after Jane and Elizabeth marry, but not Mary? Is this because Kitty is marriageable, and Mary isn't?)