Is Narcissism a religion?

If someone actually believes they're God, yes, that's a religion, specifically what's called a cult of personality. Your "or" situation doesn't hold up. Self-importance, no matter how extreme or deluded, does not a religion make. Religion is characterized by a belief in the supernatural. There's nothing supernatural about self-importance.

Moreover, most narcissists actually have low self-esteem, don't think they're God but think they're dirt. Their over-inflated sense of self-importance isn't characterized by thinking they're awesome but thinking they're awful, as they imagine they so focus on themselves that they take notice of specific parts that they then start to obsess over and distort in their self-perception that they find themselves to be extremely ugly or disgusting, so much so that the sight of them must ruin everyone's day, that people around them are repelled by them and judging them for being ugly or fat or stupid or whatever their hang up is that they obsess over about themselves. The reality, of course, is people in general aren't paying attention and aren't doing any of that because that person is not that important, because that person isn't someone people around generally spend much if any amount of emotional energy focusing on because they're focused on their own stuff. It's these narcissists that make up the lion's share, the woe-is-me narcissists, the join-my-pity-party narcissist's.   

No, they are just nuts.

What deity is worshiped in narcissism?    Case closed.

It is for NCWJ - it's what he lives by

more than anything else...

It's only a religion to the narcissist him or herself. Trump is a good example of that.

It must be. Most religious people seem to think they are a god. 

Yes, Me-ism