Currently I'm sending off 2012 by doing a marathon of the anime Bodacious Space Pirates. The protagonist, a high school girl, continually addresses her mother as "Ririka-san" and only once called her "mom" when the girl was announcing an important life-changing decision. The relationship is good between the two so the casual address is apparently not meant as a sign of disrespect, but I'm unsure what exactly it's supposed to indicate. Were this a fictional American girl habitually addressing her mother by first name then I would understand certain things about that mother's background, her parenting style, and the relationship between the mother and daughter, but I don't know how the practice is supposed to be taken by the anime's original Japanese audience.
My question, then, is what does it mean when a Japanese high school girl continually and without hostility addresses her mother by first name?