When you drive through mud, sand, or dirt, nothing gets on your tires/frame. I would like to pose an idea of having a dirty texture fade in or something like that when you drive through dirt, mud, or sand. Second, when you go through water, all of that dirt would wash off, making your car look nicer!
iirc Neilogical did a YT video covering a BeamNG clone for mobile that does the gradual increase of dirt and mud on the car, wheels and tires too. Not sure if it washes off if you drive in water. I imagine the BeamNG devs might add something like that, albeit simplified for performance, for when the game nears 1.0. If not, there's always SnowRunners if you want that kind of thing.
