The Lycian League using Marquis/Marquess as the default title for all its members doesn't really fit. A Marquess is usually the title given to a noble who has land on the border of the country/kingdom, which is called a March. So a Marquess is a literal edgelord. Some of the Marquesses in FE6/7 seem to land on the border, but many of them are inland as well, so they would not be marquesses by this definition.
It always bothered me that they used this particular title over say Duke or Count/Earl/Jarl, which would make more sense, for a league of nobles in a loosely associated country. History buffs may have examples of the title of Marquess being used differently, but I find this title is being misused here.