“Does sunlight affect astigmatism?”

No. However a good pair of sunglasses are helpful for any prescription.

Sunlight cannot affect your astigmatism which is present due to your corneal and lens shape, and your axial length (how long your eye actually is in 2 directions). Sunlight can affect the skin near your eyes, your eyelids, and can affect your retina. Also, UV radiation can increase the effects of getting cataracts sooner than expected.
So, protection is the key here with wide-brimmed hats, sun-screen/sun blocker with high UV protection, and sunglasses with UV protection, always.
Good luck and best wishes.

Sunlight has vitamin D and that has affect on the cornea