I don't think that cruelty is implied in Lydia at all. Certainly she's selfish, but in a teenage 'The world only exists insofar as it affects me' way, which hasn't been 'civilised' out of her by training in good manners. Like when the Bennet women are visiting Netherfield while Jane is ill, Lydia just sees it as an opportunity to try to inveigle Bingley into holding a ball. Literally her only occupation seems to be being man-mad, with occasional excursions into hat-trimming and card games.
I think the oddity with Lydia is how different she is to all her sisters, despite the age gap between youngest and eldest being small (seven years). Jane and Lizzy are comparatively accomplished, intelligent, elegant young women, while Mary is well-read and practices hard at her music. Presumably they've all had the same educational opportunities, as described by Lizzy to Lady Catherine, Lydia (and possibly the coughing nonetity Kitty) has just not taken them.
But what is far odder is not that Lydia doesn't play or sing or have ladylike manners, it's that she doesn't seem to have got the memo about 'morals', female reputation and sexuality that her sisters got from somewhere. I mean, surely we don't imagine that Lizzy and Jane were just as boisterous, over-sexed and thoughtless at fifteen, but grew out of it? Who taught the older Bennet girls not to throw themselves at passing men, and not to elope/have sex until married? If it was their mother, why didn't Lydia get the message too?
(It always reminds me slightly of that scene in one of Antonia Forest's books (which take religion seriously) where tactless Lawrie betrays over a family teatime discussion of the school nativity play that not only does she think Jesus/Mary/God the Father etc are all imaginary, she thought literally everyone else thought so too, and when someone asks her why she thinks they go to church, she looks blank and says 'Hymns and things.'