Hi Quenten
Can I suggest using lots of tiny mountain symbols rather than the mountain fill. I think it would be clearer, and worth the extra effort. The problem with the fill when its used on at such a tiny scale is that the pattern is a bit repetitive, and the tops/bottoms/sides get shaved off the mountains all around the edges of the fill. You could make the polygon(s) larger and use an edge fade, but that wouldn't really solve the problem.
Or you could use a lighted bevel effect with the mountain background instead, as has been done on at least one of the continental maps of the Profantasy world back over on the Profantasy forum. In fact that would probably look even better than thousands of tiny mountain symbols, and take much less time