We think Mrs. Johnson is totally hilarious. She's like the nosey neighbor in almost every sitcom that ever aired. Much like Kramer on Seinfeld, she has a real skill at getting free food out of her neighbors, the Youngers. She's only onstage for a few minutes and she manages to bum some coffee and a piece of pie. Pretty slick, Mrs. Johnson.
While the Youngers' nosey neighbor definitely provides some comic relief, she also brings a darker tone to the play. She carries with her a newspaper that reports that a black family, living in a white neighborhood, has recently been bombed out of their house. This news totally raises the stakes of the Youngers' upcoming move and adds a lot of tension to the play as a whole.
Even though Mrs. Johnson seems friendly on the outside, she also seems to kind of resent the Youngers. She insinuates that they think they are "too good" to live in the mostly black neighborhood anymore. Mrs. Johnson almost seems to enjoy sharing the information that a black family was bombed by racist whites.
When she leaves the paper in the Youngers' apartment on her way out, it's almost like Mrs. Johnson is implying that, if the Youngers find trouble in their neighborhood, then they're only getting what they deserve. Essentially, Mrs. Johnson represents the feelings of resentment that some blacks felt when others started to climb the socio-economic ladder.