Ok so I was watching this video here:

And at about 6:04, he used the rocket nozzle glitch to get into corona mountain early. Is there a reason this isn't used to beat the game really quickly? (Also, it's been a couple years since I've played Mario Sunshine, so i don't remember exactly when you get the rocket nozzle and all that)

The loading zone isn't present until all Shadow Marios in every world is defeated. This has been known and that guy did it NG+ so no.

Aw ripp. Well that sucks.