Much like dragons are color coded for your convenience, evil tends to be ugly for your convenience. Usually. There are exceptions of course. The seduction based monsters tend to have the features of attractive humans. But you'll never see a good creature that looks like this freakish beast. Good can be ugly. That runs towards the cute ugly though. More disney hunchback of notre dame than tragic burn victim survivor. Definitely not anything in the nightmare fuel category. Which really is a shame. Imagine if there was a flip around. The ugly disturbing creatures were the compassionate understanding ones and the shining beings of light were the downright sadistic assholes. It would certainly throw most D&D players for a loop. The monster manual is proof enough of that.