I noticed this feature has been added to Preview application for a while. But as it didn't take MTL files into account, I thought it wasn't so interesting.
After your post, I tried Preview application again, but remembered that it's sandboxed, the reason why this application wouldn't have the right to open MTL and images if the user doesn't explicitly open them with Preview too. Therefore, I selected all the files exported by Export to OBJ Format menu item (OBJ+MTL+images), dropped them on Preview icon, and hurray, it worked this time! [:)]



(but with no transparencies under Mojave)