I can only speak for myself and my observations, but I don't think it's normal. It happens, but it's uncommon. I think it's more normal for people to have similarly attractive partners. Some people are cuter than others even at the same "level."
Most of my girlfriends have been about the same level of attractiveness. I think I'm probably a 5 or 6 out of 10. If I lost weight and cleaned up I'd probably get to a 7, but that wouldn't change my attraction to my wife. Maybe at that point it would look like I "settled"?