...laundry?
100% Cotton
"Oi, Usopp, you done with my shirt yet?"
"Yeah, here, Zoro."
They all have their chores, and Usopp's chemistry knowledge comes in handy for more than making shooting stars. He figured out how to get tabasco stains out of his overalls long ago, but that's nothing compared to the challenges the crew presents.
"Hey, Usopp, thanks!"
Not only do most of Nami's clothes require special treatment, but there's ink spots to deal with. Or grease stains on Sanji's suits--what kind of idiot cooks in silk? And no one could possibly imagine what Luffy manages to get on his clothes. Much less how.
"By the way, Long-nose, good job with the laundry."
Zoro's shirts, however, take extra particular care, beyond the vinegar and ammonia concoction he's developed to get blood out. But he's got it down by now. Boiling water, then use the pedal-operated tumbler to whip it dry, though he has to be careful not to overdo it; Nami charges for mending tears.
"Thank you for doing the laundry, Usopp. It came out great."
As an artist, he understands the importance of aesthetics in maintaining good spirits. And as resident storyteller, he considers it his responsibility to keep up the ship's morale.
So if Zoro's going to cover himself with a shirt, the least Usopp can do is make sure it's a size too small. Or two or three.
Now if only he could figure out what to do about the pants.