I look at Splatfests as a ranked battle that isn't competitive.
Winning does actually matter, but it should be at the bottom of your Slosher list (as opposed to bucket list). Your priority should be to have a good time, and splat the night away with a free mind. Treat it like a ranked battle, and fight to win, but don't worry if you lose. You get rewarded even if your team loses, and just as the ink vanishes after the battle, your failures will too. With ranked battles, you'll have to claw your way back up the ranks if you fail too many times, but nothing carries over for Splatfests. Enjoy the music, get loopy looking at the lights, and remember the most important thing: It's all the best in a Splatfest.