The answer to 'why is my grass mud' is that your grass is likely becoming waterlogged due to poor drainage or excessive irrigation. Grass can turn to mud when the soil is oversaturated with water, preventing proper root growth and causing the ground to become soft and mucky.
Some common causes of muddy grass include heavy rainfall, overwatering, poor soil drainage, and compacted soil. Symptoms may include standing water, squishy ground, and grass that is difficult to walk on without sinking. To solve the problem, you'll need to address the underlying drainage or irrigation issues.