] I feel JA is being ironic here- by making Lizzy, who though is pretty herself, is only second best in looks, is not a favourite of her mother, and is slighted by a very eligible young man at the ball-the heroine.

I've always felt that Elizabeth was not her mother's favorite because she is the sister most like the father (and her father's favorite as well). Elizabeth's superior intelligence must be intimidating and frustrating to a woman such as Mrs Bennet, who was probably "a silly girl" like Lydia when she was in her teens. Mrs Bennet was apparently very attractive when she was the age of her daughters (Chapter 2)and may have been experiencing some unconscious rivalry with her least-favorite daughter, who is considered "quite a local beauty," per the snide remark of Miss Bingley.