I've found over time that one man's utopia is another's hellish nightmare. This is true of every utopia and should be a pretty strong argument against implementing them at all.
People who have all their needs met (food and safety, social, etc) tend to want to make the world around them better. I would wager the nice looking places are more likely to have well-treated people.
Places that look like dumps are the ones more likely to be populated by people who are treated poorly.
TLDR; they built a mouse utopia universe 25 with plenty of food water and space . Eventually some grew violent, others sat and groomed themselves all day, they stopped breeding and died out.