By its nature, a concept like Zomia cannot be "real" in the way that I think you mean. Nobody living in the region it identifies would ever have identified it by that name. Zomia is an attempt by western scholars to describe and classify documented patterns of behavior exhibited by many people. It is both a social and geographic relation between cultures that are sedentary and urbanized "developed" states. The concept of Zomia is used chiefly in reference to southeast Asian highlands but James C. Scott, a noted Zomia enthusiast, applies the same phenomenon to a number of other regions and periods including the fertile crescent from the outset of the neolithic revolution onwards. Cultures that develop around cities and depend on agriculture do not move themselves very easily. The commerce of necessary goods for urban life, chiefly cereals, requires well-maintained roads or suitable waterways, military outposts to protect it, administrative networks to organize all these functions, and labor resources, be they soldiers, road workers, rowers, etc. Especially for a developing state, the logistical challenges of this kind of project would limit its scope in predictable and specific ways. Steeper and more remote areas anywhere are much more difficult to build roads into, and once built they are still much more difficult to move goods in and out of, especially using draft animal power. In his brief and useful book "Against the Grain, a Deep History of the Earliest States," Scott describes how early kingdoms in China would levy taxes payable in rice, but could collect them only within the distance rice can travel by wagon without spoiling.
Returning to the concept of Zomia itself, Scott and others explore this outer boundary of statecraft from the other direction. Instead of a disappointing failure by early administrators to extend the bounty of civilized life to people living in "marginal" areas, they contend that cultures might consciously adapt themselves to ways of life like hunting and gathering or mobile livestock pastoralism specifically as a means of remaining out of reach, living as refugees from the state itself. This is the point where the "real"-ness of Zomia ceases to be an empirical question. Its use in scholarship attempts to get at the motivations of people who lived on one side or the other of this boundary, which I hope I have successfully characterized as both real and imagined at the same time. Famine, drought, war, or, Scott says, debt crisis, might at one time drive people in urban centers to "head for the hills" while a few generations later economic prosperity in cities might lead people to move in the opposite direction.
To succinctly answer your question, yes Zomia is most certainly a useful concept. It is mostly a real, observable phenomenon. Still, the fact that western historians and anthropologists have described something does not make it real per se, especially when we are talking mostly about transhumance by people, and about about their culture and ideology, often in the absence of written source material where these people could have characterize their intentions for themselves. Good initial reading would be the James Scott book mentioned above or another of his, "The Art of not Being Governed." Hope this helps!