Nope. What you’re experiencing is one of two things, it’s either a ‘bad anodization‘ process with the 6000 series aluminum on the 6S and/or it’s from the sweat from your hands is acidic where it’s causing a reaction with anodized aluminum (Notice along the sides of your picture, that’s the worst area because that’s where your hands are consistently touching.)
Also, there were some minor issues with the anodization on the 6000 series aluminum, but it was very rare, [and alleged to be a small batch of iPhones that actually experienced this]. However, the ‘Fading’ from the OP‘s phone compared to yours are two entirely separate issues.
Here is a thread showing the exact situation with another member with the Apple Watch:
Apple Watch pitting corrosion?
This is my Apple Watch bought in April 2016. Love this watch but I started noticing spots about 6 months into use. I used to think that aluminium didn’t corrode. Could this be corrosion from sweat?forums.macrumors.com
Im hoping the "aerospace-grade" aluminium of the iphone 11 end ups being better, as i usually prefer the phones caseless (at home at least).