Since a pin up girl is a girl that you put up on your wall does that mean all girls are pin up girls?

Not necessarily. Pinup modeling refers to Photography that is meant to be pinned up on a wall such as calendars and posters. If a model has not appeared in such publications then they are not really considered "pinups." Additionally, there is a specific style that is associated with pinup modeling that includes the 1940s-1960s style (hair, makeup, outfits) that fall under the pinup category. It is a specialized field that not all models meet the requirements for.