Context #1: 

Cameron and Dave are
friends. They are talking about a party.

Cameron: I’m so
excited about our high school party!
Dave: Really? Why?
Cameron: I asked Julie
out and she said that she’d be my date.
Dave: Hmm.. that’s
cool!
Cameron: What?
Dave: Nothing.
Cameron: I don’t even
see the point of you guys going out together. It’s not like you are going to
stay together or anything. You are going to two different colleges in two
different states.
Dave: Man! Sometimes
you are such a Debbie Downer.
Cameron: I’m just
being realistic.
Dave: Actually, you
are just being negative.

Context #2

Melanie and Valerie
and are co-workers 

Melanie: I’ve worked
at the flower shop for 5 years.
Valerie: Do you like
it?
Melanie: It’s okay.
Valerie: Just okay? We
have such amazing benefits and our boss is so nice.
Melanie: Yeah, but
sometimes the AC doesn’t work and we don’t have a place to park our cars.
Valerie: Well don’t be
a Debbie Downer and stop looking at just the negatives. Think of all the
amazing positive things about the job!

Meaning – Someone who says something negative and ruins a positive environment. This idiom comes from Rachel Dratch’s character, Debbie Downer, on Saturday Night Live.

 https://languagesystems.edu/