To that stranger on the KOOL Tree House chat room, I gotta hand it to you: You were, ultimately, not wrong.
In this comic, Cueball, very likely as an autobiographical representation of Randall, describes to Megan the first time he interacted with a group of people unknown to him over the internet; in an AOL chat room for children called the "KOOL Tree House chat room" in 1993, when Randall was about nine years old. He read a discussion about Green Day, asked who they are, and was told that not knowing was a serious problem. As Megan says, judging people for lack of pop culture knowledge has remained typical online behavior.
Green Day is an American rock band formed in the East Bay of California in 1987. In 1993, they were still known merely as an independent punk band, and a year away from releasing their major-label debut album Dookie, their first mainstream success. Anyone, especially a nine year-old, not recognizing the band in 1993 would be perfectly normal. After 1993, Green Day would go on to be a widely popular and influential rock band with many acclaimed albums.
This initial online social interaction was a significant formative experience for Cueball, molding his online behavior ever since, in that it still causes him to consider his correspondents' perspectives when communicating. The social dynamics at play are reminiscent of the mathematics of others' perspectives described in 1053: Ten Thousand. Relating the personal experience of an oversized effect from a casual insult is humorous because the extent to which early experiences affect people can be both ironic and profound.
The title text indicates Cueball agrees with the reply to the question. This is humorous because it is effectively Cueball admitting that something is very wrong with him, possibly coincidental with and unrelated to knowing that specific piece of trivia. Alternatively, Cueball could have come to believe Green Day is culturally important. If this story is autobiographical, Randall could have hence become a Green Day fan, or at least acquired more than a passing knowledge of their œuvre, recently mentioning their song "American Idiot" in 2665: America Songs.