Some good news. I recently found ways to make the mud better (more realistic) and I decided to share them here if anyone is interested and doesn't already know about this. And I'm talking about both visual and physical properties.
To remove those oversized ugly mud chunks from tires:
In the game's installation folder, remove env_mud_particle from MeshCache.zip.
To make mud stickier and more realistic:
In the game's installation folder, extract the file media.xml from Media.zip and change the value as follows, then put it back into the zip file:
<Mud>
<Substance
FullPushDepth="0.9"
LinResistance="1.8"
PushResistance="36.0"
SubstanceFriction="0.3"
/>
</Mud>
In _templates\trucks.xml, use these values. This mainly increases pavement traction to reduce the excessive wheel spin that MR has on pavement, while not allowing vehicles to climb ridiculous slopes.
<WheelFriction>
<Offroad BodyFriction="1.5" SubstanceFriction="1.2" />
<Default BodyFriction="1.5" SubstanceFriction="0.75" />
<Highway BodyFriction="1.5" SubstanceFriction="0.5" />
</WheelFriction>
Keep in mind mods typically have their own wheel friction values that will override these, and they are often too high. I recommend to remove them and go with the template values instead. See one of the stock wheel xmls (in classes\wheels in media.zip) for an example of what it should look like.