It isn't just the mother who is nameless, but the grandmother as well. The only female to have a name is the female child, June Star.
Not naming the mother symbolizes that she is, as you point out, a non-entity. We see this in her behaviors as well as her lack of a name. She is a fairly passive woman who does all of the wifely and motherly duties she is supposed to do on this long drive, like feeding the baby. However, once the Misfit enters the picture she may as well not be there. Although her husband and her son Bailey are shot she hardly protests. Then when her own time comes to be taken out and "join her husband," she just steps out of the car without any protest, her arm sort of falling passively behind her. She walks out into the woods with no fight. So, her lack of real identity, as a result of her submissiveness and timidity, matches up with her not being given a name.