Okay, so how did we get here?
The current background noise of America is Christo-fascism. The Religious Right have seized power so doesn't it make sense the worst criminals around would seize upon the opportunity to scam people religiously? If you start your own church, that's a license to print money especially in the US. So yeah, someone like the Joker would eventually figure that out.
But the real seed to this idea started when I discovered Filipino Batman. In the late 80's, around the time Batman '89 came out, the Phillipines released this absolute gem of a movie, Alyas Batman and Robin. The final ending is a musical number, wherein the Joker proclaims he's healed and "let's be afraid of God" as a show of his change of heart. That's honestly the most maniacal turn of phrase The Joker could ever say. The Snyder Cut could never.
Remember, America is now run by the religious right (and given that whole church tax exemption thing, likely always has been). Using religion to wreak havoc on people has devastating consequences. It's only natural we'd see a corrupted preacher like The Joker. I'm just shocked it hasn't happened already.