If you're asking about how widespread the faith of Nagakabouros is, I'm pretty sure he/she/it is only worshipped in the Blue Flame Isles.
But if you're asking about Nagakabouros' power compared to other deities in the Runiverse, then that's more complicated.
Honestly, I don't think that Nagakabouros exists in the same way that someone like Aurelion Sol exists. Surely, the forces that drive Runeterra are infinitely more mysterious than human minds can percieve, and so the Blue Flame Islanders, a people with a close connection to the ocean, take a sea-serpent as their idol.
Remember A Sol's taunt to Illaoi? "You do not understand your god."
Because Sol knows more about the universe than any mortal. He knows where Illaoi's tentacles get their energy from, he likely knows what happens to a soul after death, and where a soul comes from before birth.
If you ask an islander any of these things they'll say the answer is a big squid, because in that culture, Nagakaborous represents these mysteries. But other cultures will interpret it differently.