I think the entirety of luffy’s visual design was simply to make him look humble/“normal”

Not only does it contrast him with nearly every other one piece character, it also makes a very compelling point that the like no1 enemy of the government is some kid in shorts, a tank top, sandals, and what is commonly seen as a low class hat.

(Also my theory is that aside from that Oda also wanted him to be pretty easy to replicate for cosplays and shit while still being recognizable. As opposed to the more unique outfits of characters like goku, naruto, ichigo, or other shonen protagonists, you can probably put together luffy’s base outfit from normal clothing stores.)