I agree it's not really accurate to call it "The World's Ugliest Piece of Music." That would imply that the beauty of music is solely a function of how repetitive it is, which is obviously false because a single note repeated at regular intervals (or maybe a square wave held indefinitely on a single pitch) would certainly not qualify as "The World's Most Beautiful Piece of Music."
However, I don't mind this as a bit of marketing. There's really no denying that this piece is intended to be listened to after a brief explanation of the mathematics behind it, and "Mathematically Ugliest Music" is a more intriguing hook than the more descriptive "equally-tempered 88-tone row without repeated pitch or interval classes, played so that no two notes are rhythmically separated by the same rhythmic distance, on the premise that repetition is necessary for beauty in music."
While this piece does minimize a positive aspect of music, it does nothing to maximize negative aspects. Dissonance counterpoint comes to mind as a better example of actually trying to write unpleasing music. Basically, it takes the rules of counterpoint theorists have used to describe the music of Palestrina or Bach, then slavishly follows the opposite of those rules.