In one of your earlier posts, you mention that Mamma's funeral service was in the LDS chapel, so I'm assuming she was a Mormon. What are your thoughts on why she is Catholic in the children's books? Because it was easier to tell the story that way?In short the answer is yes.
Phyllis Fogelman, the publisher for The Great Brain books, wrote a publisher's note at the conclusion of the postumously published, The Great Brain is Back. She gives a wonderful explanation of how this delightful series came to be. The Great Brain book was originally intended to be the fourth and final book in the adult series. However, by the time John had written it, the market had changed and no one wanted to publish it. Eventually Fogelman read it and saw the potential as a children's book. She said,
Soon after I came to Dial, I read the manuscript and told the agent I would love to publish it - if the author would cut the novel in half, add some bridging passages and delete the few references that were aimed at adults rather than children ( and were by far the least interesting parts of the book.).... I took the three versions home and spread them out on a large table in my small dining room, which I kept closed for weeks while I edited The Great Brain....It is unclear to me who made the decision to present the entire family as Catholic in The Great Brain series. In 1969, in preparation for the next book in the series, Phyllis wrote John with a request, "I'd love you to add something some place in the first half of the manuscript about the Mormon-Catholic split in the community."
From that I gather John made the decision to change Mamma's religion to Catholic, but I can't be sure.
If and why he made that choice I can only conjecture. The first three adult books came out in the mid 1950's. Seventeen years would pass before The Great Brain, the first book in the series was published. This would mean a completely new audience. No guarantee's that readers from 17 years earlier would read these books, and clearly the youth crowd wouldn't have read Papa Married a Mormon. Because every book needs a tight conflict. (A force of evil to overcome, it you will.) The clearest "evil" would be to be the family of the outcast religion in a community. Though Price/Adenville were actually quite unbiased in their religious base. Most readers would easily assume any Utah town would be predominantly Mormon and therefore apposed to Catholics. If Mamma had been Mormon readers might wonder why other Mormon's weren't nicer to her kids. That's my guess as to how Mamma became a Catholic.