What if anyone that does worship her inevitably becomes an undead somehow? Like maybe it's part of the bet that any living mortal who worships her is farted to become undead.

She said she's had clerics among the undead and just most of them were killed, it's possible that "literally no one has ever worshipped her" was referring to living people, but that's not literally no one imo, so it seems like hyperbole anyway.

It could be that undead belief and worship aren't particularly nourishing.