Does it make sense to speak of Jane Austen’s characters being true to their own selves?

To a large extent, most of the characters in Pride and Prejudice act according to the social mores of their time. This is a rigidly structured society in which everyone knows their place and must strictly adhere to certain standards of conduct. In acting out their social roles, people are required to put on a mask which they show to the outside world, but which hides their true selves. Only very occasionally does the mask slip.
Lydia Bennet is a rare exception; with her, what you see is what you get—a flirtatious, vivacious young lady with an insatiable zest for life. She has personality to spare, and asserts it forcefully at every available opportunity. This has its risks, though, as can be seen in her sudden decision to elope with Mr. Wickham. Yet say what you like about Lydia, but at least she's always true to her self, and that's more than can be said of most.
The character of Mr. Collins is interesting in that his true self neatly corresponds with the role allotted to him by society. He plays the role of an oily, ingratiating curate, obsessed with the niceties of social rank and position. But this is not just an act; this is exactly what he is behind the mask of the respectable country parson. Whereas the existing social system forcibly discourages the dissolute behavior of young ladies like Lydia, it reinforces the rampant snobbery and shameless social-climbing of Mr. Collins and his ilk.