
I think what it is is her mom was British and for a few years while Thalia grew up, they were in Britain, then she moved to LA to try to be a movie star. Thalia still has her British mom and years growing up with that so she stuck with the accent, but Jason could have been born in the US, so maybe he still has an American accent. It would also help with the Roman/Greek dichotomy even though it’s not even close to being mentioned in the books
I mean, I don't think it's the biggest deal, but it does require you to suspend your disbelief and not ask questions. thalia was fifteen and living on the streets fighting monsters, she wasn't making it across the pond during that time so there isn't a world where she's just actually british