Any mountain that lets you ‘jiggle’ at the bottom can be climbed.
What I mean by jiggle is when you hit the highest you can go, but instead of just standing there, the game shows you climbing up a couple inches and sliding back down, over and over.
Scope out a mountain until you find these spots, they are common. position yourself facing the mountain and rotate your camera so you are looking almost straight up. Click the sky above the top of the mountain. It SHOULD drag you up there.
2 things usually happen:
1) you clicked too far from the top of the mountain and end up running backwards, just try again
2) you did it right and warp a whole ton, then appear up top.