The answer to 'why is my deck mud' is that your deck is likely experiencing drainage issues or poor soil conditions, causing it to become muddy and saturated. This can happen due to factors like heavy rainfall, poor grading, or a high water table in your yard.
Some common symptoms of a muddy deck include standing water, soft or spongy spots, and excessive moisture buildup. To fix this problem, you may need to improve the deck's drainage, install a gravel base, or even rebuild the deck with better materials and construction techniques.