In response to the complaints of all the girls not smiling, the girls in the center of the photo (where your eyes are drawn to first) are all smiling or giving some expression of the sort. As you move outward the expressions change to more stoic sorts. Not only are your eyes drawn to the center of the picture naturally but subconsciously your eyes focus on the happy expressions at the center as well. If you didn't notice the girl with the best rear and the girls with the best faces are towards the middle of the picture as well. If every single solitary girl was giving the same smiling expression i feel that the photo would be extremely redundant, and I'm glad the photographer chose to shoot this photo this way, as opposed to the way the commenters are suggesting. If you want to complain about the photo not being a true pinup, then you could say that pinups don't contain multiple girls in close proximity in one photo, traditionally. So noting that fact, it's easy to ascertain this is a modern take on the style and thus some deviations are customary.
Usually pinups are smiling...in this pic only 3 of the girls have an actual pinup expression (or what I'd expect of a pinup) while the others are... just there Nice colors and photo, I don't say otherwise, but I think the whole posing and expressions are pretty much off.