it's been a great tool to show beginners, imo. it clearly demonstrates the effect/power of CSS on the aesthetics of a site. the fact that some examples are "so ugly" can be used to convey that impact...
The thing you need to remember is that CSS Zen Garden was big around 2004 and most of the designs are from that period. The site was a big inspiration at the time to show that with a single HTML file, you could completely change the appearance of a site. This was huge!
So the downvotes are likely coming with a thought along the line of "how dare you show disrespect for that venerable site!".
tangent: should downvotes be based on "how dare you?" sentiments?