I'm from Spain. There's a song with this title so I don't know it this is the reason you are asking. It does mean literally sweet girl and it's used for men and it's overly romantic, only used when you're in love with a girl. And typically from south Spain, it's rare to hear it in the north where I live.. . Anyone can use, however, expressions like: que niña tan dulce (what a nice/sweet girl) on a normal basis, like to your neighbor's daughter. Not widely used in spoken Spanish as it's a little bit old-fashioned but may be common in songs, poems,....