Well, yes and no. Lizzy may trail behind Jane, Lydia, and Kitty in her affections but Lizzy is also admired throughout the neighborhood for her looks and her spirited nature (the latter of which Mrs B doesn't understand). Mrs B wants her daughters to be married, so I don't feel that her reaction is that unusual for any mother in her position. I would be more surprised if it had been Mary: the reaction might have been different, as she herself isn't above insulting Mary in public.