As a yoga teacher, I see many yoga mats at different stages of cleanliness and degrees of wear-and-tear. Some people bring in brand new mats that have never been unrolled, while others bring in obviously well-loved, well-used mats that have seen better days.
Some are dirty from extended use over time, or from dirty bare feet and hands over a short period of time. I can tell if a person has pets if the mat has claw marks or pet hair on it.
I also keep a good supply of yoga mats at my studio, for students who prefer to use those, and it's my responsibility to clean them and make sure they're in good shape.
If you keep your yoga mat clean and store it properly when you're not using it, it will work better for you and it will last longer. Dirty mats don't have as much stickiness to them, and mats that are carelessly stuffed in a closet while not in use may develop wrinkles, creases, or gouges, making them less usable.
Here are a few methods that I use to help me take good care of our studio mats.