The name Mietta is a girl's name . Mietta and discussed in our forums with posts like "Picky Parents BNG".
From the experts:
Mietta is either a diminutive of Mia (which in the tradition of many Romance language diminutives is longer than the original) or a "smoosh" name combining Mia and Etta. As combined names go it is elegant and fresh, and avoids the sensation of having been made up.