Is there a way to change the density or thickness of mud in the world editor? All I know how to do is use the terrain painter and that just paints the terrain of course.
Density or thickness can't be changed per map, but you can try playing with the values used for all maps in lua\groundmodel.lua. If the tires don't sink into the mud at all, you'll have to add depth with a .ter.depth.png that matches the terrain. It can be independent of the terrain materials, but it's easy to match with Export Terrain Heightmap in the editor and then use the exported textures to make the depth map in Photoshop or some other image editor. See the depth maps in other terrains for example, white for solid ground and light grey for mud etc. Make sure it's in the right format or it won't work.
Abusing this thread for another mud related question if thats ok... Can i reload the ter.depth.png while the map is open, so i dont have to reload the complete map from the menu? ...after i loaded the menu itself, as its a map too ...
I would try just going to file in the editor and open your level.mis again, (save first obviously if you need to) that way you don't need to load the title screen/map each time. Works for most things so I guess it should work for this. Unless it needs a total reload of the whole game for the settings to take effect. (doubtful)