G'day Rompy,
I will also refrain from mentioning the enormous splif he's holding and instead concentrate on the mud effects.
Firstly, congratulations on taking the bold step to weather your figure with mud and in doing so cover up some of the paint work underneath. It's something some figure modellers just can't bring themselves to do. Having taken this bold first step, let me encourage you to take this process to the next level.
Firstly, consider the life of a soldier in the field on campaign and the build up of dirt, mud and grime that will accumulate on his clothes and personal equipment over an extended period of time. Consider too how this build up of mud and grime will be most obvious on those parts of his clothing that will most often come into contact with the ground.
In the case of your figure (a machine gunner) he will be firing his machine gun mainly in the prone position (lying on his stomach, and supporting himself with his elbows). It stands to reason that his elbows, chest, abdomen and the front of his legs down to his knees will be pretty dirty and probably wet and muddy. His bum will also be muddy and wet from sitting down on the ground. Over time his clothes will start to show the cumulative effects of being constantly dirty with wet fresh mud (which is typically darker in colour) caked on top of older dried mud (which tends to be lighter in colour).
Hands are also in contact with the ground and his mittens should be almost black with all the accumulated grime and crap he's handled.
His face will be streaked with mud and dirt and sweat and the pores of his skin will be clogged with grime. Every time he wipes his face with his hands he's transferring dirt from his hands to his face.
If I have one criticism of your mud effects is that they seem to be all the same colour, and lack variation. I think you could enhance the look of your mud effects significantly just by adding some darker patches on the elbows, knees, bum and on the lower abdomen, and some lighter dried mud elsewhere. This is relatively easy to do. Just with your paints, create a base colour for your mud and then add some white or buff colour to create your highlighted mud (for dried mud) and add some black/ raw umber to create a darker mud (for fresh mud).
Another suggestion would be to put some mud on the feet of the bipod for the MG and on the bottom of the stock. Most soldiers would keep their weapons much cleaner than they would keep themselves, but in battle such considerations are often neglected.
I would certainly encourage you to keep trying different mud effects and experimenting with different materials. I use a combination of pastels, real dirt and pigments and use different forms of application. Sometimes I flick the mud on by dragging my finger over the bristles of a toothbrush to create a splatter effect, other times I just slop it on with an old paintbrush to create a thick layer of mud that will form a dried clump. What ever works for you.
The bottom line is that combat soliders live on the ground and that effects how they look.
I hope this helps.