So, quick story. I was on the phone with Debbie P. (my blessed, holy mother) a while back, and I made mention of someone being a "Debbie Downer" and she stopped me and goes, "WHERE did that come from?" And I laughed and laughed. She continued "I feel like I'm a really positive person, so I don't think that applies to me." And I continued to laugh and told her that I thought it came from an SNL skit, but I was met with resistance (when I retold this story). But, per Wikipedia, Debbie Downer DID, in fact, originate on SNL.
However, I made a vow to my mom that I would do my DAMNDEST to eradicate that term, or at the very least remove any reference of her name when attached to DOWNER.