Simple and Direct:
* Covered in mud
* Mud-caked
* Muddy
* Soil-stained
Descriptive:
* A thick layer of mud clung to their clothes.
* Their skin was splotched with brown, like a messy canvas.
* Mud dripped from their hair and clothes, leaving dark trails on the ground.
* They were a walking testament to the day's adventures, every inch of them plastered with mud.
Humorous:
* They looked like they'd been wrestling a giant mud monster.
* Their clothes were a new shade of brown, courtesy of the mud.
* They were a walking, talking mud sculpture.
* A mud-covered mess, but with a smile on their face.
Figurative:
* Their clothes were stained a deep ochre, like a forgotten sunset.
* Their face was a mask of mud, hiding their true features.
* They were as earthy as the ground they walked on.
Choose the description that best suits your story and the overall mood you're trying to convey.