What nationality is the last name Dowell?

This interesting surname of Scottish and Irish origin and derived from the Gaelic personal name Dubhgall, composed of the elements "dubh" meaning "black" plus "gall", "stranger". This was used as a byname for Scandinavians, in particular to distinguish the dark-haired Danes from fair-haired Norwegians.

Is being a German name?

Dutch and North German: patronymic from the personal name Bein, a reduced form of Bernhard.