As I recall from seeing her speak right as the final episodes of season 7 were airing, she needed an accent but it couldn’t be her british accent because they wqnted it to not match Picard! This was noticed as being inconsistent when we met her family: she didn’t speak like her mother so it must be her father’s accent. Well, they messed up when we heard her father. So, she guesses, she must have gone away to school!
Actually, I didn’t see it as a problem. Kids pick up the accent from where they are living and their parents don’t. So the accent is from where they lived when she was acquiring language, different from where her parents grew up.
But growing up in different places and having parents with different accents allows her to have a mixted or even inconsistent accent, as she had multiple influences. And over 7 years she talked more like the others on the ship.
Anyway, out-of-universe, 25 years ago I don’t recall her telling me any specific accent it was based on. Just that she tried different things that she could speak consistently and the producers OK’ed one of them.