Social stereotypes are cases of metonymy, where a subcategory has a socially recognized status as standing for the category as a whole, usually for the purpose of making quick judgements about people. The housewife-mother subcategory, though unnamed, exists. It defines cultural expectations about what a mother is supposed to be. And because of this, it yields prototype effects. On the whole in our culture, housewife-mothers are taken as better examples of mothers than nonhousewife-mothers.
The characteristics of a given social stereotype may or may not have much basis in fact. Stereotypes can sometimes be a relatively value-neutral categorization of behavior (e.g. the view that most parents have a tendency to nag their children) . On the other hand, when unjustified stereotypes are applied to groups, often the result is negative. Negative stereotyping is a key feature in prejudice, as racism, sexism, et cetera.