In Maggie: A Girl of the Streets, Mrs. Johnson is somewhat of a villain. She completely rules the Johnson home and is also incredibly unkind to her children. She is also an alcoholic and often makes a spectacle of her anger. She is violent with her children; at one point, she kicks Maggie out of the house for having a relationship with Pete out of wedlock, which ultimately leads to her death. When Mrs. Johnson finds out about Maggie's passing, she immediately launches into a show of overbearing grief and motherly love despite all the abuse she has shown Maggie.
In Daisy Miller, Mrs. Costello is certainly not as cruel as Mrs. Johnson. However, she does also have a penchant for theatrics. Snobbish and seemingly very proud of how her class status makes her better than those in classes beneath her, Mrs. Costello is constantly prone to fainting, headaches, and overall dramatics when something doesn't go her way. She announces that she needs to lie down and uses smelling salts and camphor to revive herself. She especially dislikes Daisy and the other Millers, acting as though she simply can't tolerate them because of their position on the social ladder. When she first meets Daisy, she calls her a "dreadful girl" and never accepts her. We can compare how Mrs. Costello treats Daisy to how Mrs. Johnson treats Maggie: with total disdain, although Mrs. Johnson was far more harsh.
Overall, Mrs. Johnson and Mrs. Costello both seem to have an acute sense of their social class and are quick to turn their noses up at others. Consequently, they both balk at scandal (for example, Maggie's extramarital relationship with Pete, and Daisy's love for attention and overall being a flirt), and react strongly when faced with it. Finally, they are prone to being overdramatic; Mrs. Costello will often declare that she is ill and make a show of going to rest until she's well again, and Mrs. Johnson is often described as shouting, pacing, or even acting out in extreme sadness (which is likely not genuine).