Put simply; the author believes that his intended audience would already know everything that could be said from an iconographic deconstruction of the ethnographic cartography of Macedonia. (Why the author thinks this is a separate mystery.)
To break it down:
"An exercise" in this context means (somewhat sarcastically) "something done or performed as a means of practice or training".
"stating" refers to the act of making a statement (ie "saying/writing something").
"the obvious" refers to things that are either easily discernible or already well-known by the author/speaker's intended audience.
So putting it together: The author would consider his 'deconstruction' no more than practice talking about things his audience should already know or be able to easily figure out for themselves.
To get a little meta: Explaining "an exercise in stating the obvious" to a native-English speaker would be an exercise in stating the obvious.