The girl next door is an archetype. It means a reasonably pretty, reasonably smart, nice girl with the same background as you, the same values, well-grounded (stable, rational). If you look up "wholesome" you'll find that describes the girl next door.
It's often used in contrast to a woman who is exotically beautiful but either not very nice or not very smart, perhaps even a schemer who uses her good looks instead of her mind for advancement or relationships.
The girl on the track team with the pleasant smile and the ponytail and not a lot of make-up is the girl next door. She's not the one who wears eye liner to the pool or expects people to do things for her.
So this model has wholesome good looks, not exotic beauty. It keeps her from thinking that the world owes her everything simply because she is beautiful. A person who is down-to-earth is practical and has the right sense of how people relate to each other - she doesn't think everything revolves around her.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.