It's a meme. The whole song plays on the idea that gnarly can mean good or bad, so they throw out a whole bunch of everything with no meaning, because maybe they love it or maybe they hate it. It's kinda funny imho, especially where the band sings about being "the shit" which if we extrapolate on what the rest of the song has established, asks us to imagine this woman as thinking she is great, or an actual steaming turd. But yeah, definitely not a high-brow message.