cosmology
Explanation
The Joke
The comic is titled "Funtime Activity: Applying Cosmology to People." A person with glasses tells another man named Frank: "According to the Copernican Principle, you, Frank, in particular, aren't special." The Copernican Principle in cosmology states that Earth does not occupy a special or privileged position in the universe -- there is nothing unique about our location. The comic takes this cosmic-scale principle and applies it with brutal directness to a specific individual, turning a philosophical statement about the universe into a personal insult.
The Humor
The joke works by collapsing the scale of a grand cosmological principle down to a petty interpersonal barb. The Copernican Principle is meant to describe the universe's indifference to any particular vantage point, but when aimed at a single person, it becomes an elaborately justified way of telling someone they are unremarkable. The framing as a "Funtime Activity" adds an extra layer of absurdity, suggesting this is meant to be an enjoyable exercise rather than the existentially crushing put-down it actually is. Frank's disgruntled expression sells the landing perfectly.
References
The Copernican Principle is named after Nicolaus Copernicus, who proposed that the Earth is not the center of the universe. In modern cosmology, the principle is generalized to mean that humans are not privileged observers of the universe -- our location, our planet, and our solar system are not special compared to any other.