The mud stain on Peggy's dress is considerably different from when it happened than when she later confronts Andy in front of his house.
The second time Peggy appeared in the series, she took care of Andy and Opie in Bee's absence, assuring they had good meals, cleaning up and singing on the porch after dinner. Opie appeared to considerably enjoy her presence. Here, in her fourth (and final) appearance, Opie suddenly doesn't care for her one bit. Although they eventually form a friendship, Peggy is never seen again.