Reminder that that these are my personal observations on each style and I do not claim that these are the definite definitions. Also this is coming from the perspective of a mori girl, so please keep that in mind as you enjoy this series. ^^
"The distant cousin from the city that sometimes goes hiking in the mountains"
I personally am not a fan of yama girl, but mostly because I so often see them compared to mori kei as being so closely related. In my opinion, the styles are completely dissimilar. The concept might be similar, mori girls in the forest and yama girls in the mountains or forest, but the styles are very different.
Yama girl includes bright neon colors, wind breakers, exercise gear, backpacks, and hiking shoes. Nothing about blending into the forest or being from the forest, no air of mystery or fantasy.
I have seen a few well-done yama coords that have an air of natural kei, which I definitely prefer and think is a very cute way to go hiking. The pictures below would represent that cross-over. Plaid is also a popular pattern, whether you're doing a cross-over coord or not!
Do you like yama kei? Have you tried it yourself? If you have, let me know in the comments! (I'd like to be convinced that yama kei is not a rave party for hikers, heh.)