Why didn't anyone think of blindfolding themselves to protect their eyes from the ugly?
I think it's because it wasn't seeing the ugliness of the barnacle that killed them, it was being in the presence of it, whether they actually laid eyes upon it or not. It was just that ugly.
Because they didn't know it would be that ugly. Everyone who saw it died and wasn't able to spread information.
Was the barnacle ALWAYS ugly? At what point did everyone die, and how long did it take? Did the barnacle's ugliness kill everyone at the exact instant that he was born, or was it something that happened later in life and more gradually?
He was so ugly, everybody killed him. Then the fumes from his corpse killed everybody.
"Everyone" could include himself.
By "everyone dying", this would include security guards, water purifiers, farmers and other people. The Ugly Barnacle would of eventually starved to death since no one would be able to manage resources for him, causing his stomach to eat itself and become too weak to live.
I was reading this to help overcome my ugliness like the story was advertised to do, and it didn't help at all! What gives?
No matter how ugly you are, you'll never be as ugly as the Ugly Barnacle. That's how it's supposed to help.
Well, think of it this way: at least no one is dying because of your ugliness.
Or it was a Class 1 Apocalypse, and only a certain region of Bikini Bottom was left uninhabited.
See above about how "everyone died" in a story generally only refers to the important characters, not literally every single person in the Universe.
Maybe everyone only meant all sentient living things died, leaving out single-celled organisms (bacteria and protists aren't usually thought of as on the same level as a multicellular organism). Life then re-evolved from these survivors.