Here's why:
* It's the subject complement: The sentence is missing a verb, but it's clear that the speaker is making a statement about the coworker. "Dork" is what the speaker is saying about the coworker.
* It's a person or thing: "Dork" is a slang term for someone considered foolish or awkward. Nouns refer to people, places, or things.
While "dork" can sometimes be used as an adjective, it's functioning as a noun in this sentence.