I can understand where that "Spyderco knives are ugly" sentiment comes from, although I don't share it.
There are two different ways of finding beauty in something. Pure aesthetics, and the beauty that comes with incredible functionality. For example, let's look at motorcycles (since I've recently become a motorcycle aficionado). I may look at a motorcycle with smooth, flowing lines, good curves, a sleek look, and a balanced appearance, and say, "Damn! That is a beautiful bike!". And it is. It may have a weak engine, poor handling, bad ergonomics, and substandard breaks, but that doesn't change the fact that it is absolutely beautiful. At the same time, I may look at another bike. It looks heavier, bulkier, with fatter tires, straighter handlebars, a wider seat, and bigger, more obtrusive instrumentation. And I may say, "Damn! That is a beautiful bike!". I may even claim it's as beautiful as the first bike. Now, it isn't nearly as pretty in a purely aesthetic way, but because I understand the functionality, and I can appreciate the beauty that comes with a well designed tool, I can see a level of beauty in the second bike.
The same is true, I think, of Spyderco Knives. Their design trademark is the hole on the blade for opening the knife. This hole, in order to work, forces a different blade shape than you see on almost all other knives. It's going to be wider, it's going to be less symmetrical, it's going to look bulkier, and weirdly shaped. Especially when the vast majority of knives people see do not share this shape. From a purely artistic aesthetic beauty perspective, Spyderco knives will fare poorly compared to traditional knives.
However, those who have held, and used, a Spyderco knife will often remark on how amazingly useful that little hole is. I know personally, I've never found a thumb stud that comes anywhere close to the spyderco style hole for ease of opening and closing a knife. Once you understand that functionality, you start to look at the hole differently, and you begin to see a functional beauty in it. Same with the black plastic handles found on so many Spyderco knives. When compared to modern synthetics like micarta, G10, or Carbon Fiber, they look cheap, boring, and plain. When compared to classic materials like hardwoods, bone, stag, etc, they look cheap, boring and plain. However, the FRN handles are very cheap to make, provide a relatively good grip, even when wet, are very durable (practically unbreakable), and are impervious to liquids, weather, corrosion, and many/most solvents. It's not fancy, but it works really well. There's a beauty in that.
"The car may not be as pretty to look at, but once you pop the hood, you realize it has a beautiful engine", so to speak.
And all of the above is without even getting into personal preferences, personal experiences that shape our tastes and perception of beauty, etc. ;-)