Is she an Eight? That’s what immediately pops in my head.
She’s so outspoken. Social embarrassment and decorum are not part of her character. In Austen’s world this makes Lydia clownish and prone to ruin. In our world she might be hailed as a confident woman, even an iconoclast. I can see an Eight fitting into either description, depending on the context.
Obviously she’s physical. She loves to dance. Austen presents this as part of Lydia’s boychasing, shallow temperament, but what if she’s just a Body Type who enjoys movement? I can imagine that an Eight, especially the baby of the family who’s been spoiled, would have a hard time meshing with the strictures of Austen’s Regency society.
Lydia’s the hero of her own story. To her delight she moves into society at a young age, finds many dances and beaux, travels away from home with a dear friend, and meets her future husband after much adventure. It’s very Eight-ish to be oblivious to the worries of rules-oriented people (such as her sister Elizabeth, the Six).
She loves her dear Wickham. He’s charming, witty, and handsome. For someone as cognizant of social rules as Wickham, though, Lydia must be a painful companion. Her vivacity will only go so far. When Austen linked them together, I believe she was punishing Wickham in devious ways. He grew up on Pemberley, associating with lordly people. For him, to be forever tied to Lydia — whose freshness would not be welcome in that era — is a come-down.