If you describe someone’s hair as matted, you mean that it has become a thick untidy mass, often because it is wet or dirty. (Collins Dictionary)
Covered with a dense growth or a tangled mass: (Dictionary.com)
Formed into a mat; entangled in a thick mass: (Dictionary.com)
Formed of mats, or of plaited or woven material. (Dictionary.com)
Twisted into a firm, messy mass. (Cambridge Dictionary)
Dog Covered With Matted Fur Is So Happy After His Makeover.
Acute hair matting.