Symbolism, Imagery, Allegory
There are a lot of nuns in this book. In fact, the priest and the nuns—or some combination thereof—appear on nine different pages (7, 9, 17, 18, 19, 21, 26, 29, 31). So what are all of these religious figures doing in a story about a magic pasta pot? Simple: being Italian and making the magic okay.
Since he grew up Irish-Italian-Catholic, you can bet the church was a big part of Tomie dePaola's early life. And in Italy in the Middle Ages, which is where dePaola set his tale, the Catholic Church was a major influence. So, on the one hand, by including the figures of the priest and the nuns in his illustrations, dePaola is simply depicting authentic life in an Italian village at that time. But there's more to it than that.
One of the first things dePaola mentions about the priest and the sisters of the convent is that even they visited Strega Nona because of her magic touch (7). And when Big Anthony serves dinner to the village from the magic pasta pot, the priest and the nuns are among the first in line (19).
But wait—isn't witchcraft considered a bad thing in many religions? Exactly. So, by including religious figures and explicitly stating that they make use of Strega Nona's services, dePaola is actively stating that Strega Nona's magic is okay. Not to be feared. Perfectly safe and innocent, and condoned even by leaders in the church.
Of course, including these folks in the book hasn't stopped people (like here and here) from challenging the story for promoting witchcraft, but still, dePaola's message seems clear. Yes, Strega Nona is a witch, but she isn't a threat to religion, order, decency, kindness, or faith.
At least, that's the way dePaola sees it.