So I'm not too familiar with how BotW is built, but from the looks of it they are not using an overall macro normal map, which would be sculpted per rock and section that gives the overall rock its shape. Instead they are using micro normal details (so a tiling bumpy rock surface) that tiles across the entire rock wall area. And because of the way the game functions and works, they are basically using designer geo (very simple shapes that the player interacts with 1:1), where a lot of modern titles would create a tile set of fully sculpted rocks with Macro and Micro normals/materials, that they then would be the wall out of modularly. But since the climb surface has to be pretty specific you cannot rely on that density of surfaces that may obstruct the player path and be very difficult to test.
There are ways around making a set locked off climable wall that then you could use a set of to make more detailed climable sections that have a higher fidelity, but they didn't seem it important to priotize it, which makes sense.
This is it. To me it looks like they were stuck with a relatively basic terrain editor. Individual assets like trees, buildings, and stuff like that look great, and so do scenes that are made up of those elements, but once you get into less handcrafted areas it becomes a lot more rough, and it looks like the tools they had to modify geometry, paint textures and decals and so forth were pretty basic. (Look for instance at the floor from straight up, and in many places you can see paths and patches of an artist's soft brush painting textures)
It's not a matter of being "Wii U game" or the Wii U being too weak, but rather a result of the tools they used. Let's not forget it started development very early in the life of the Wii U, and they were probably stuck with those tools lest they break something else, whereas more modern Nintendo games have terrain that looks a lot better. I'm pretty sure this is something that will be addressed in the next game, because it's the biggest sticking point in the visuals of BOTW. The climbing system is already pretty solid in tricky geometry, so I'm sure they could make it work. Already some rocky scenery looks great (Death Mountain) whereas others look terrible, especially when rock intersects with grass.
It also doesn't help that the scale of the mountains in this game is gargantuan.
