I don't know if it's meant to be a joke, per se. They use (or used) the Unicorn server once upon a time:

https://github.blog/news-insights/unicorn/

https://news.ycombinator.com/item?id=4957986

I don't think it's a joke, it's the server that github runs on

https://en.wikipedia.org/wiki/Unicorn_(web_server)

One of Reddit's cutesy error pages (presumably for Internal Server Error is similar) is an illustration that says "You broke reddit". I know it's a joke, but have wondered what effect that might have on a particularly anxiety-prone person who takes it literally and thinks they've done something that's taken the site down and inconvenienced millions of other people. Seems a bit dodgy for a mainstream site to assume all of its users have the dev knowledge to identify a joking accusation.

Even if it is their server name, I completely agree with your point. The image is not appropriate when your multi-billion revenue service is yet again failing to meet even a basic level of reliability, preventing people from doing their jobs and generally causing stress and bad feeling all round.

I am personally totally fine with it but I see your point. Github is a bit too big for often braking with a cutsey error message even if it is a reference to their web server.

That stupid "Aww, Snap!" message I think it's one of the browsers does.