excellent first attempt i would say.
2 points:
1: I suspect there would be more build up of mud on the sides of the tires and in the hubs. it sort of looks like they have recently been splashed with muddy water rather than dredging through the quagmire represented on the base.
2: the demarkation on the base between muddy road and clean foliage is far too crisp - mud splashes and spray from the wheels would quickly cover the nearby plants. plus, drivers rarely plunge through the thick of the mud once conditions get like this - they tend to start driving on the side, and so the road gets broader and the sides get prograssively muckier. think of a dirt path that is muddy - no one continues to walk through the mud but tromps on the side to varying degrees. plus you have mud sloshed up on the vehicle, so why not the surrounding ground cover?
anyway, promising technique there, and i shall give it a go sometime.