I think that developers should be able to do design, because often there is no other choice. Not all projects can afford a specialized designer.
As a developer I feel like my design skills are modest. But I strongly believe that features are more important than great design. For example, I think that many people would agree that neither Craig’s List or eBay has great design. But they are very successful sites.
I’m not sure what “design principles” are. There’s a fad for little cartoon icons and a cartoon esthetic for a lot of sites. I completely reject this esthetic, at least for nderground. We wanted something edgy. Hence the black, grey and white design theme of nderground. We’re also designing for adults, not children.
A core design goal for nderground has been to provide an experience on mobile devices that is as good as possible, without providing a custom app. This is much more important than whether our colors or fonts are just right (what ever that would be).