Answer may be:

Firstly, the Yoga mat should be cleaned once a week as weekly cleaning requires less effort.

For light dirt fill a small bottle with warm water and dish soap. Spray the water all over the mat and wipe it with a damp cloth. This will not only help in removing dirt but also the foul smell of sweat.

If the yoga mat is very dirty then soak it in a tub of warm water and a detergent solution or dishwashing liquid and leave it for an hour. This will help dislodge dirt, oils and any odours.

Avoid using too much detergent as it can make the mat slippery. Now gently scrub off the dirt with a  soft brush and rinse it with clean water.

Let it dry for a while in the sun.