As the title of the novel suggests, Main Street in Gopher Prairie plays a central symbolic role in the book. When Carol Kennicott first sees the town, all she sees is that "Main Street with its two-story brick shops, its story-and-a-half wooden residences, its muddy expanse from concrete walk to walk, its huddle of Fords and lumber-wagons, was too small to absorb her" (4.2.2).
In other words, Carol is not going to be happy with her new life, because Main Street is simply not enough for her. This is supposed to be the heart of the city where all the interesting stuff happens, but the sad truth is that Main Street is filled with a bunch of fashion-impaired people with no real ambition in life, at least according to Carol.
This is kind of a bigger problem than it might seem, because "Main Street" is pretty much a code word for all American small towns everywhere. Do you think New York City has a Main Street? Or San Francisco? Or Chicago? Nope: Main Street is pretty much Main Street wherever you go. So the problems Carol has with this particular Main Street are problems that she has with small-town America in general, even if she can't quite articulate that.
By the end of the book, Carol has accepted that she'll probably never change Main Street or make it more beautiful. But she also promises herself that she'll never give in to thinking that it's fine just the way it is. As she tells herself in the book's closing scene: "I do not admit that Main Street is as beautiful as it should be! I do not admit that Gopher Prairie is greater or more generous than Europe!" (39.8.10).
Carol will continue to stay true to her beliefs about culture and beauty. But at the same time, she knows there are thousands of Main Streets all across America that are just like the one in Gopher Prairie, and none of them will probably ever change much.