Short of it's potential.
One, I think you're overlapping Goth with Emo. They aren't the same thing.
Goth = Black-wearing, 'outcasts' of society, the like being creepy, and can be downright hostile to anyone no in their clique`
Emo = Suicidal, people who are manically depressed.
Two, there SOOOOO many goth stereotypes to poke fun at, and all you got to was the black clothes?
Three, the humor was bland, just a bit of name-calling.
Four, the "SMB1: Defeat Bowser/Our princess is in another castle" scene has been done more times than the only whore in a seaport. There are DOZENS of other topics in the Mario universe.
Five, (this is kindy nit-picky, but w/e) The actual part where Bowser falls into the lava is Untrue to the Game. There's supposed to be a Ceiling, and the Bridge is a little bit below the Ledge with the Axe on it. (Between 1/2 block to one block above depending on the Level, if my memory serves)
Six, Too short. Even if the joke had been good, it's at best a one-trick pony. Hell, you coulda slipped by if you had a fair number of Mediocre jokes. More jokes = Better score
Seven, the Voice samples needed cleaning up. Static and Background noise was present. A no-no when using ripped sounds.
Eight, (last one) Mario's black clothing was invisible against the black background of the castle., I know that's how the game is, but you could have differed the two by a few shades, so it was visible.