I think I debunked this theory before on YouTube (it was a comment) but let's do it again:

1. The Mushroom Kingdom is literally next to the Cap Kingdom. It would make sense as to why Mario got there of all places.

2. It's a mechanic in the game and something that can be done when tossing Cappy onto a possessable creature.

3. I haven't found anywhere that says that the number 14 is associated with karmic debt or whatever, only personal freedom. Also, religion doesn't matter in this universe.

4. According to a Redditor JJDude, and I quote, only in the Chinese Chang-E lore does the rabbit makes a medicine for immortality by pounding medicine in a mortar; nothing was said about the medicine being able to bring the dead back to life. In Japan the rabbit is up there in the moon making mochi, which is a popular new years food made from sticky rice.

5. Once again, according to JJDude, the "return" is meant for returning money, not to bring back from the dead.

6. Yeah, let's not act like they were in the Mushroom Kingdom all along.

7. Once again, religion doesn't matter in the Marioverse.

   A bad theory, but at least it's better than the "Mario is a psychopath on drugs" garbage.