My cousin doesn't exactly baby his console, he's pretty young after all. Still that shouldn't happen within such a short amount of time. Maybe after years of heavy use that could be expected, but even then a quality product optimally shouldn't do that.
I think it might be because the metallic paint on the n3DS XL doesn't adhere very well to the plastic. None of my 3DSes have had that problem (o3DS, o3DS XL and small n3DS), not even the o3DS which did use metallic paint, but I guess it's a different brand or something.
Plastic is rather tricky to paint to begin with, people doing custom paints on plastic devices will tell you that you need to sand the surface, apply primer, then apply the paint itself, otherwise the paint won't stick.
I usually put crystal cases on my consoles, all my 3 3DSes have had a crystal case for most of my time owning them which obviously helps a lot in preventing this sort of thing. They're cheap and give the consoles some protection if I want to throw one in my pocket without the pouch or something. And they still fit in the pouches with the crystal case on, so I see no reason not to get crystal cases.