The answer to 'why is my sprinkler system mud' is that a common cause is poor drainage in the sprinkler system. When the sprinkler heads are not properly installed or the system is not sloped correctly, water can accumulate and create muddy conditions around the sprinklers. This can also happen if the sprinkler heads are placed in low-lying areas that don't have adequate drainage.
Other potential causes include broken or clogged sprinkler heads, leaks in the pipes, or issues with the water pressure or timing of the system. Muddy sprinklers can also be a sign of a larger problem with the overall drainage in your yard.