Well... if you look at the stitching on Mario's overalls in Melee, you can see a rectangular label on the back that is brown and looks like a Levi tag.
I don't think their overalls are denim. Seem too stretchy and whatnot.
Now what I wanna know are what brand of shoes. I saw a man a few years ago that was wearing, no joke, the same EXACT boots Mario wears. It had the stitching on the side that went straight down, then the stitching next to it that went down, 90º right for about an inch I'd say, and back down. It had the same curves and the same exact cut soles and tread that Mario's has. I almost walked up to that dude and asked him what brand his shoes were so I could get some, but I decided against it as to not look like a complete idiot.