Nathaniel Hawthorne never actually refers to Hester Prynne as an "angel." In fact, the text also does not come out and say explicitly that the "A" stands for adultery either. Those are left up to the reader to come to that conclusion. The only specific "A" word that Hawthorne associates with Hester is "able."
Angels stereotypically do things like encourage people, help them out of trouble, provide for the poor, do miraculous things, etc. Along those lines, Hester can be seen as an angel of sorts. She does give to the poor in the novel, makes clothing for the poor, she sits by people and cares for them in their final moments of life, she is constantly encouraging toward Dimsdale and even forgives him for what he did to her. Hester is an angel through her good works in much the same way that Mother Teresa can be considered an angel.
In the Scarlet Letter, one of the greatest form of symbolism can be found by the letter A Hester Prynne wears the entire time. This is how she is characterized as an angel. Within later in the book, the townspeople see an A in the sky and view this as angel. Those who are reading the book would believe that the A stands for Hester as she is isolated and shunned even though we do not believe she is at as much of a fault. The A also begins to form supernatural characteristics that represent how she can be viewed as an angel. Hester Prynne was very prominent and soon the townsfolk started to believe they saw her go in and out of cottages and places as if she is a myth and this ties in to how angels were similarly viewed as a myth. Moreover, Hester does many good deeds which can tie in to how Hester can be viewed as an angel. I hope this helps you!!