Steampunk times? When exactly were steampunk times? You do know that's not a real historical time, right?
What is a Hawaiian shirt but a button-up shirt with bright colors with a floral pattern (usually)? Totally within the realm of possibility for them to produced by Elysian textile shops.
Also, if you don't buy that, I've established that temporally-speaking NeOlympus takes place in modern times, they just live on a world that's a bit behind the times & Hermes still visits Earth. As Hermes is like the ultimate tourist, he's probably the sort to buy Aloha Shirts in bulk. Not a stretch that he'd give some to Dionysus.