One of the ongoing jokes among fans of "Beavis and Butt-Head" is how visually distinct they are -– even when compared to the other characters on the show. Everyone else in their universe seems to have roughly the same proportions as a real human being. Beavis and Butt-Head, by contrast, have long, bulbous heads, completely out of proportion to their tiny, caricatured bodies, as well as exaggerated Robert Crumb-esque facial features that border on the grotesque.
In "Beavis and Butt-Head Do the Universe," when the wormhole in 1998 spits them out on a beach in 2022, the space-time anomaly of course attracts the U.S. government to investigate. When government agent Mattison (voiced by "Harvey Birdman: Attorney at Law" star Gary Cole) looks at a sketch drawing of Beavis and Butt-Head, he immediately says, in a grim tone, "Whatever they are, they're definitely not human."
In fact, Mattison believing Beavis and Butt-Head are actually part of an advanced alien species is a running joke throughout the film. This includes a scene later on, when he hears a conversation between the two on an intercepted cellphone call, and states (again, grimly), "It's crude, but they're learning to approximate our speech."
It's the metatextual acknowledgment of Beavis and Butt-Head's outrageous designs, on top of Cole's committed delivery, that sells the humor.
