I have a terrain with two materials going into a Mix Shader, I want the fac input to be the slope of the terrain. Steep parts should be one color, flat part another.
You can easily blend different shaders based on the angle of an object's face.
You can get the face's angle using the Normal output of the Geometry node. Separate out the axis you need (in my case the Z axis) using a Separate XYZ node. Then I use the ColorRamp node to fine tune the range I want the transition to happen in. Do note a RGB Curves node would work just as well, it is just up to which you prefer.
Here is what those nodes look like rendered.
I have a variation to this, and depending on what you are doing, it may make sense to use one of these answers over the other.
You may need to use true normal, depending on if you are animating the object(s) in rotation for any reason, and anything in the world that would be rotated may need to keep its material selection to be at the top of the rotated object.
so same basic steps but slightly alternate node setup.
Here's The result.