Not its natural habitat - it would probably die in winter
Axolotls are somewhat popular as pets so I’m thinking someone got rid of theirs by tossing it in the river and the girl just happened to find it afterwards.
Far more plausible explanation than “found in the wild 9000km and an ocean away from its place of origin”
They freeze and thaw like Iguanas do in Florida. They can’t survive prolonged cold temperatures but when it does get to 15c they stop moving.