In "Half and Half," by Amy Tan, what does the metaphor mean when the narrator writes, "My mother believed in God's will for many years. It was as if she had turned on a celestial faucet and goodness kept pouring out"?

It's a way of expressing An-mei's simple faith in God. An-mei firmly believes that her faith is responsible for all the blessings and good things she's received in life. The faucet metaphor is very useful, because it perfectly encapsulates a comforting, uncomplicated understanding of what faith in God entails. Just as when we're thirsty, we turn on a faucet to get a drink of water; when An-mei wants to enjoy the blessings of life, she places her unconditional faith in God.
But as her faith is based largely upon an almost constant flow of blessings, it's inevitable that when the "faucet" is turned off, then that faith will falter. And that's precisely what happens in the case of An-mei. For when Bing, her youngest child, falls into the sea, An-mei implores God to give her son back to her. Sadly, her prayers go unanswered and An-mei loses her faith.