That's my understanding of buzzkill (and by extension, buzzkiller), too.
Imagine a celebratory party going on at a college campus after the basketball team wins an important game. A student with no interest in sports walks in and says, "Don't stay up too late, guys -- we've all got semester exams on Monday."
That's a buzzkill -- spoken by a buzzkiller.
Similar phrases would include killjoy, stick-in-the-mud and party pooper, although buzzkill(er) in particular conveys a sense of putting a sudden damper on the happy mood.