Earlier quoted context omitted.
When someone is agreeing you already know what they are looking for as they've expressed it in their agreement. When someone says something is ugly you don't know what they think is ugly about the design so it doesn't provide any thing but a comment.
No, I often don't know what it is that makes people gush about the beauty of multi faceted things like operating systems, gadgets or APIs. But be that as it may, I doubt this is what causes the downvoting. I'm pretty sure a statement like "colors and icons are ugly" would have provoked a similar reaction.
Saying "X is ugly" is not much use and is mean, thus more likely to attract downvotes.
Saying "I think X is ugly. Here's why {list of subjective reasoning}" may be useful and is not as mean.