丫头 literally means “Y shaped head” and is used to refer to little girls. This is because in the ancient times, the little girls used to wear two plaits with one on each side of their heads and thus made their heads looked like an upside down Y.
傻丫头 can be translated as “silly girl” which means funny and cute girls and is an affectionate form of address to the little girls by their parents, aunties and uncles.