Last time I put mud on a model, I used a mix of backing soda, and Tamiya’s Flat Earth color. When this was mixed, I threw in some cut up fibers from some twine I had lying around. I got a very nice muddy texture, mixed with what looked like dried grass.
I also have an article in which the author used brown paint, filler, and cut up brush bristles. This looks very good on his model.
One other technique I have tried is putting on filler before I start painting using a stiff bristled brush. I just tapped the filler on with the end of the bristles to give it texture. Then painting the mud over areas with filler on them once the model had been painted the base color. Dry brushing especially brings out the little chunks. BUT this method looks more like old dried on mud and dust rather than just wet mud.
Good luck with whatever method you use, and I look forward to seeing any other techniques used by modelers on this forum. Oh, yah, and please let us know what method you used, and how it worked for you.