Mariana is the play's official nosy neighbor. She doesn't get much stage time in this play. In fact, she only appears in Act 3, Scene 5 but, boy, does she make a big impression. We're talking, of course, about her reaction when someone mentions Paroles' name. (She yells out "I know that knave, hang him!" [3.5.2]. We sort of get the idea that she really means it. If Paroles were in fact hanged, Mariana would probably have front row seats.)
Aside from that, Mariana's main job in the play is to warn Diana that sex-crazed guys like Bertram can't be trusted (3.5.3). In fact, her name reminds us of Shakespeare's other Mariana (the jilted woman in Measure for Measure who teaches bad-boy Angelo a lesson by helping pull off a bed trick).