I don't think it's an insult anyway. I like the clean and simple look, and the Windows 95 theme gets a lot right in terms of UI design. Generally high contrast, easy to tell at a glance which widgets are clickable.
Hacker95 seems to get the Windows 95 buttons wrong though, showing them as 'sunken', and not giving a clear impression of being clickable.
It's a joky insult. Hacker News and Windows 95 have in common that they look objectively old. But what they also have in common is that they are function over form in an atypically pure sense, especially compared to some modern UI design.
Although I prefer a more balanced approach on the spectrum of pure function and pure form, pure function beats pure form any day
Hacker95 seems to get the Windows 95 buttons wrong though, showing them as 'sunken', and not giving a clear impression of being clickable.