Lorde - Swingin Party Lyrics Meaning

That's actually a song by The Replacements. Lorde's cover is a decent one but the original has a melancholy which can't be touched. Paul Westerberg is one of the greatest and most underrated songwriters in recent history and I encourage you to explore the entire Replacements catalog as well as Westerberg's solo work.
Now, what is "Swingin' Party' about?
Paul Westerberg has spoken about this song quite a bit and said its about a few different things. It's about what it seems to be about superficially, yes, but also touches on times when you (the generic you) are so uncomfortable in social situations that you (the generic you again) need to self-medicate with alcohol and fake happiness to make your way through the Swingin Party.
He said the band needed to do this often with respect to performing. They were all incredibly uncomfortable getting on stage and drank...a lot...before shows. It's a relatable concept for people who have social anxiety or stage freight. They may not use substances but many people have coping mechanisms, calming techniques, and yes, substances, which help them in uncomfortable situations.
It's about depression, losing yourself in a crowd, hiding in plain sight, sadness and longing disguised as joy. It's about whatever you need it to be about. It's songwriting at its finest.
"Bring your own lampshade, somewhere there's a party."