... more than she worries about Lydia's morals is not good mothering. And Mrs Bennet didn't do any more than Mr Bennet to instill morality into Lydia. Lydia's mother is at least as much to blame for Lydia running off with Wickham as her father is.

Yes, the Bennet daughters had to marry if they were going to be supported after their parents died, but Mrs Bennet is thrilled that Lydia is married at all, and at a young age. She thinks that Wickham's charm and good looks make up for the fact that he seduced and took advantage of a stupid teenager. Mr Bennet at least understands that he has to accept (some of the) blame for Lydia's future, even if he isn't likely to change his own behaviour.