They may have some theological distinctions but they aren’t related to the plain/fancy distinctions at all. Holiness/methodists are culturally extremely mainstream and Methodists have always been at the forefront of uniting church with “popular” worldly culture, as opposed to the separatism (in particular of culture down to dress and technology) of plain people. Holiness/Wesleyan people are “fancy.” So are almost all contemporary Quakers and many or most mainline Mennonites.