>>115415819The joke is that instead of writing a traditional joke featuring a setup and punchline (or a witty one-liner), the writers instead put the words "Joke About Coffee", implying that there should be a joke about coffee following the above format present.
However, there clearly is no such joke, and your expectations of the presence of one are subverted by the words on the shirt. The subversion of expectations is, itself, essential to many brands of humor, which turns the self-aware "Joke About Coffee" into a meta commentary on humor which is, itself, funny.
I hope that answers your question, Anon.
