Does ECC RAM make a Pi-hole noticeably more stable?

No, ECC RAM won’t make your Pi-hole noticeably more stable. DNS queries are small, simple operations, and a bit flip hitting the exact right data at the wrong time is astronomically unlikely. Most Pi-hole setups run on ARM boards that don’t support ECC, and building a custom x86 server for it is total overkill. Spend your money on a good power supply and a healthy SD card instead.