Hi cjl62281 (my what a catchy name!
) and Rebecca
It can be so frustrating to be foiled by your own sweat. 
Ideally you don’t want to use a towel to wipe sweat for any other reason than if it stings your eyes. But of course if your grip is REALLY*** compromised then sure, either wipe it off once, or place a single layer of towel over your legs so that you can grip your legs or hands or whatever you can manage.
***Some students do use a towel when they don’t need to, and then they can’t seem to drop the habit easily. Case in point it is rare to really need a towel for ANY standing pose. If you are someone who routinely uses a towel for anything other than lying or standing on, then drop it and see how you go. Get used to practicing without it and before long your practice will quickly strengthen. If you have to use it then go ahead - be very discriminating.
Namaste
Gabrielle