Hi Jacob P.,
The mtl file itself is not loaded at all. It is a file that contains data, which is taken from the file to configure the import.
When you get an obj and an mtl file, there might be a texture file as well, if that is in the mtl file referenced.
Open the mtl with a text editor to see what it going on.
For example, the “map_Kd” (without the “”) is typically the line for an image in the Color channel.
This can be provided with a name only or with a path. If it is with the name, only the texture needs to be in the target folder where your C4d file lives.
If it has a global path like /yourhardrive/yourtexfolder/your_image.jpg then it knows where to find that.
When you get an obj/mtl/image file from someone else, typically, that entry is only the name.
Please check the file, open the C4D in the folder, then merge the obj file. Does the cube have texture?
All the best