Lainey seems rather brainless and ditzy to me. She would be shallow and self-centered and very fond of having her own way (read spoilt brat) however she would probably be one of the popular 'teen queen' sort.
I don't think it's popular at all in the UK (I've know one and it was an occasional nn for
Elaine) however it does strike as the sort of name the 'chav' population would find ever-so-cute, I can see a little Lainey-Mae with siblings Alfie-Kai and Demi-Leigh or something and being friends with
Madison,
Alisha and all the 'American' sounding names. The sort of name that never gets really popular but keep cropping up in
BAs, usually with a hyphen - Lainey-Jo, Lainey-Mia, Lainey-Rose etc.