Daemonettes of Slaanesh do not rely on their physical looks to be beautiful because they are surrounded by a psychic haze that changes the perceptions of those they encounter.
Their true appearance matters little, however, because they are surrounded by the bewitching aura of their Master. This supernatural power makes them always appear as the ultimate beauty and object of desire in the eyes of their unfortunate enemies, regardless of their race, gender or morality.
- Codex: Chaos Daemons, page 33
They are ugly because they need to be adept at combat, for they are also the front line warriors of Slaanesh's armies. Their physical bodies need to be able to swiftly despatch enemies when they are not using their other talents:
The Daemonettes are Slaanesh's warriors and messengers beyond his realm. [...] The Daemonettes are vicious, merciless fighters who attack with astounding speed and grace, tearing apart their foes with sweeping artistic strokes of their razor-sharp claws.
- Codex: Chaos Daemons, page 33
