All yoga mats begin to show signs of wear and tear, even with regular cleaning. Rubber mats are prone to visible marks and dirt build up, and you can't remove scratches from PVC mats either.
But you can continue using your mat with some cosmetic damage, though it's time to replace your yoga mat if you start to find your practice harder because the non-slip surface has degraded or there are tears.
If you do hot yoga or enjoy a sweaty practice, your mat may begin to smell, and unless the mat has an antimicrobial layer, there's not much you can do to keep that in check without damaging the surface layer, impacting the grip.