I don't have the original blaster illustrator file, so for now, the skull is merely superimposed over the existing blasters, when I get home, the skull will cast some shadow on the blsters and the blasters will be moved a bit differently...
Great start. I think for this to go over well you'll need to change the skull to fit in to the cartoonish nature of Mario World. Perhaps use a dry bones skull or draw what the skull of Mario would look like.
I like it on a lighter color (It's hard to want to buy a white shirt) but it would look good on any light color. I do think that your lightest gray color could go lighter. It makes the whole thing seem kind of dull and dark.
I went overboard on the gun's tones and used up all my colors, it'd be nice to have a clean pearly white, I'll have to play around with the guns to see if I can substitute a color anywhere, or as you'd mentioned, just lighten the one tone.
Personally I wouldn't wear this shirt, but I think you have a really great design here. I love the old school NES guns. This would be a perfect gift for my brother.