On the mobile website, the icons representing each site in the inbox are a bit unpolished. It's especially noticeable on the iPhone 4's retina display. I also noticed that the Meta Stack Overflow icon seems cut off to fit the dimensions of the icon. It seems as if the mobile site is taking the icons from somewhere else, downscaling it, and using it on this part of the site (which would explain the MSO icon getting cut off and the pixel artifacts).
Perhaps it could instead use the icons used in the full site's inbox?