Notice how I said it 'can be'. Out of the box, Vim is not very attractive, but with a bit of work it actually looks rather nice (yes, I did modify the application icon, but that's not really what I want to get at here).
Some of this beauty is subjective due to the syntax highlighting used, configuration of invisibles, etc, but that's configurable and therefore can be customized to your liking.
There are clean and elegant fixed with fonts, which again, you can choose whichever you prefer.
What I meant by simple was the complexity (or lack thereof) of the UI. It only shows you some basic information that is readily important, everything else is hidden away in various keyboard shortcuts. This makes it more complex to use, but it also means the beauty of your work, the code, is at the forefront with little distractions. Hell, I even turn off scrollbars.
At the risk of starting a discussion of opinions over my choice of syntax highlighting, here's a screenshot of my vim setup:
http://dl.dropbox.com/u/18782/macvim.png
I don't really understand how you can call this ugly, unless you're referring to my taste in syntax highlighting.