It depends on context whether a 'good-time girl' is a prostitute, an escort (with or without sexual favours) or simply a girl having fun.
'good-time' has a broad meaning as an adjective. As you can see, when you describe someone as this they may be a prostitute, or nothing like that and simply someone who likes to have fun or who likes pleasurable activities.
Occasionally, you hear 'good-time' to describe a few other things too, like music. One that that is almost common is 'good-time rock and roll'. You add 'good-time' to create a sense of the 'fun times' you can have with this genre of music.