This issue is a little sticky because, for the most part, you didn't really have landed burghers in Japan like you did in Europe. The myoshu were prominent enough in provincial politics that I really don't think they should be left out, but they were in a weird grey area between noble and burgher that makes them tough to fit into CK2. At any rate, I think mayors should be considered peasant leaders rather than merchants in most cases.The problem is, Nanushi, Shoya etc. were leader of peasants, and CK2 doesn't have the representative of this social class.