It depends on the software. I wouldn't trust an encryption library which had a website with a really "beautiful" design however I wouldn't trust a CSS framework

It depends on the software. I wouldn't trust an encryption library which had a website with a really "beautiful" design however I wouldn't trust a CSS framework that had a very basic or ugly design. For a language like Go, which is aimed at more advanced developers, I would say that design really doesn't matter that much. That said I prefer the design of the Go website to all of those that you linked and that Git website still saddens me when I see it: I miss the old site :( So, yes, presentation does matter but that doesn't always mean that it should be pretty. For some audiences, that can be a turn-off.