The conversion's great and the mud's not a problem, but the painjob is a little boring and too 'Christmasy' in all honesty. Slap on a new and exciting coat of paint to this and try and keep more to metalics for the majority and paint 'overtop' on some of the planks and such in a darker colour with some sort of design I'd say. Flames on red or black background is always slick. ;) Pick out the little details moreso, too such as the gas can, glyphs, etc. You don't really need much work on your actual skill for painting Ork cars, but just on the method and design of the paintjob instead. :)
I think you went too nuts with the mud. Or rather, the color for the mud. it looks too bright and artificial, same goes with the bright greens on the machine.
you need work on the ork, do you have the codex? great ideas that you should use like drybrushing goblin green over dark angels green.
But the truk just ROX