Lydia proves herself a total airhead. Remember the letter in Ch.52 when Aunt Gardiner writes to Lizzy? She comments 'I talked to her repeatedly in the most serious manner, representing to her all the wickedness of what she had done, and all the unhappiness she had brought on her family. If she heard me it was by good luck for I am sure she did not listen...' Lydia is irresponsible, she cares nothing for her family and clearly shows she has abandoned her Longbourn life to remain with Wickham. I've no doubt she deserves a slap from her father if he'd stir himself, but what good would it do now? She wouldn't understand nor particularly care how damaging her conduct had been.