Rarity's popularity over here is all over the place. She usually starts off as everyone's least favorite pony because she seems like just a big preppy stereotype at first glance. Then she starts to grow on you and skyrockets after you realize that she is just as complex and interesting as all the others. The things that make her so awesome are more subtle than with the other characters and it takes a bit to see them. After that, she can almost do no wrong ^_^ Any time she's on the screen she becomes pure comic gold. Also she has a tendency to appeal more to artists (writing, drawing, or otherwise) for some reason so that may answer your question too.
I'm an American and simply adore Rarity! I suppose she is just misunderstood by most Americans because of the general erosion of manners and morals in this country. Perhaps her ladylike manner is just a bit too "old-fashioned".