“Fancy” is has a very specific meaning as as opposed to “plain” here and “upper class people are also fancy” is not generally true. “Plain” refers very specifically to lifestyle rules and modes of dress and sometimes speech adopted by conservative Anabaptists, most notably Amish, and historically, Quakers (very few of whom are meaningfully plain anymore although “simplicity” is still taught as a Quaker value). It has nothing to do with social class, there are many examples of plain peoples who were better off as a whole than their “fancy” neighbors. Fancy, in the context of plain people in America, is just anyone who lives according to the dominant culture or otherwise isn’t bound by the rules of plain people. Those being things like what kind of fasteners or colors you can have on your clothes, what kinds of hats men wear, head coverings and dress standards for women, etc., and for the Amish extending to bans on mains electricity, automobiles or self propelled vehicles, etc.