Wait Till the Sun Shines Nellie

The previous song is a rendition of "Wait Till the Sun Shines, Nelly" with Bing Crosby and Mary Martin. According to Wikipedia, "Wait Till the Sun Shines, Nelly" was a popular 1905 song written by Harry Von Tilzer and lyrics by Andrew B. Sterling. Traditionally, floor traders at the New York Stock Exchange sing this song on the final trading day of every year, in addition to Christmas Eve, because the song has been the stock exchange anthem since 1934.
On page 156, Kurt Vonnegut alludes to this song by stating, "The barbershop quartet on the airplane was singing 'Wait Till the Sun Shines, Nelly,' when the plane smacked into the top of Sugarbush Mountain in Vermont. Everybody was killed but Billy and the copilot. So it goes."
Ah, how tragic! The barbershop is singing an incredibly upbeat and happy tune just before the plane crash. So it goes. (And - by the way - who doesn't love a good barbershop quartet? I can only think of one person who might disagree...)
After listening to that song a few times myself, I have already fallen in love. It's such a catchy tune! I immediately picture four men in dashing red-and-white-striped shirts, mustaches, suspenders, and white hats singing in perfect harmony while the audience applauds. Ah, 1905! The "Good Ol' Days!" I remember those years like they were yesterday... What? That's just me? Oh, alright.
The question now is, what purpose did Vonnegut have with the allusion to this song? Like I mentioned earlier, Vonnegut's purpose was to give the reader the image of everything working perfectly one second, and horribly the next. Everything is seemingly status quo in the plane with the quartet beautifully singing a fun, happy song (after singing a few crude, racist tunes) and tons of optometrists recalling old times. (Side note: What could a bunch of optometrists really talk about anyways? Example: Optometrist 1: Yeah, this patient of mine was nearsighted, but I accidentally told him he was farsighted. He walks into a door because he couldn't see! Hahahaha!
Optometrist 2: Hahahaha! Great story, Jim. One time I had this old lady who was as blind as a bat, so I gave her trifocals! And even then she couldn't see! Hahahaha... It was the funniest thing, you had to be there...... Yeah, sounds like a great plane ride....) Anyways, everything is going great, until- BANG! The plane crashes and no one saw it coming. Only two survive alive. The rest... weren't so lucky. So it goes.
So that's what I think Vonnegut's purpose was with "Wait Till the Sun Shines, Nelly." He was creating a mood of liveliness and happiness until, one second later, everything changes. I can immediately picture the scene. How tragic.