In my opinion Ubuntu's desktop environment looks much nicer than whatever Xubuntu and Lubuntu use.
1. Lubuntu and Xubuntu use different desktop environments.
2. They don't look very good at the start because you can theme them yourself. Just install numix and some icons then change the desktop background. Boom, pretty desktop. The point of DE's like XFCE, LXDE, and KDE is that they are nearly infinitely customizable. Looking nice is not a problem.
3. Unity (Ubuntu's DE) is one of the least customizable DE's out there. So much so that in order to change the GTK theme you need 3rd party tools.