My OBJ files have to endure a haphazard workflow, through Hexagon, Blender, UVMapper Classic, MeshLab, etc. (back & forth, in seemingly random order)—- so I’m resigned to occasionally checking them in a text editor. (Under Windows, WordPad, since the different proggies have different line-end habits.) I locate each line that contains mtl and make sure the material names are as I intended, doing replace operations as needed. And also deleting any interlopers like that def_surf_mtl (kowabunga headbangers?) from both the OBJ and MTL files.
You only have to get really serious about this cleanup just before the final import into Daz Studio. Of course, I’m never certain which one is going to be the final one, so…