This quote is from Chapter 22 in the book. It comes when The Preacher is praying (saying grace) at the party at Gloria Dump's house.
My interpretation of this is that he is talking about how people are not easy to understand. All of Opal's friends are in some way weird. Gloria Dump has all this guilt in her past, Otis was a convict. But all of them add something to her life.
Part of the point of the book is that we have to find what is valuable in all the people (and dogs) around us even if that thing of value comes along with flaws. That's what makes people "complicated and wonderful."