It is just an accessory, you don't have to wear it, and whether it looks good or not depends on what you are wearing. With my previous kilt (a military style with quite a high rise) I hardly ever wore a belt as a good few inches of tartan would appear above it which looked slightly odd. I generally wore a waistcoat or jumper instead.

With my newer kilt (which sits on the natural waist) I quite like wearing a belt but purely because I like the look (it is just a wide bridle leather with a brass buckle not a kilt belt with the fancy buckle). It isn't tight enough to be uncomfortable and just provides a bit of balance to the ensemble.