Well, you said it already, a whole world map doesn't add up with "realistic" mountain representation.

But, then again, most maps of Earth will not have the mountains represented. If you label cities and rivers, the mountains and deserts kinda "appear" in there, because they are the large areas without rivers/cities (deserts), and the areas without cities where the rivers start (mountains)... If you then place a single label in that area, say, in italics, with Desert X, or ZY Mountains - it may completely do the trick!