The genre of Main Street can make for interesting discussion. Today, many people think of the book as a great example of satire because of the way it exposes the people of Gopher Prairie to ridicule as typical small-town Americans. But in a memoir, Sinclair Lewis's first wife, Grave Hegger, claimed that "Main Street was not a satire until the critics began calling him a satirist, and then [Lewis started] seeing himself in that role" (source). In other words, Lewis himself likely thought of Main Street as a realist novel describing the malaise of a dissatisfied housewife.
It's fair to say that Main Street might be considered either a realist novel or a satire, depending on how funny you actually find it. If you just find the novel flat-out depressing, it probably fits more into the realist mode. But if you catch yourself constantly chuckling at Lewis's characters (including Carol), then you probably lean more toward the satirical reading. Lewis's writing is both realistic and biting, so the novel really is probably a mix of the two genres, whatever Lewis's original intentions were.