This question can be successfully answered through an examination of the first chapter of this excellent coming-of-age classic, which details the living arrangements that Huck has to endure after The Adventures of Tom Sawyer. Note what the opening chapter tells us about the Widow Douglas and her ideas of parenting:
The Widow Douglas she took me for her son, and allowed she would sivilize me; but it was rough living in the house all the time, considering how dismal regular and decent the widow was in all her ways; and so when I couldn't stand it no longer I lit out.
Wen need to remember that the book is told from the point of view of Huck Finn, and thus it is tempting to see things as he presents them, rather than have a more objective view. Let us remember that Huck probably was a very difficult child to look after. Having been used to living by himself and living off the land, it would have been a massive adjustment for him to living a "civilised" life. Therefore, I think we can say that there was nothing bad about the parenting style of Widow Douglas and her sister, apart from their obvious lack of understanding about Huck and what made him tick. They seemed to take him on as a project, and the way that they try to "civilise" him involves teaching him about the Bible and Christianity, manners and reading and writing. Clearly, for their generation, obedience could be commanded through threats of what would happen to you if you were evil, as is shown when Mss Watson told Huck stories about "the bad place."
Therefore I don't think that Widow Douglas and Miss Watson were necessarily bad parents, but they did show a clear lack of understanding about Huck and his position, which resulted in the kind of erroneous conclusions that Huck makes about religion in the first chapter.