If Slaanesh is the god of perfection, the lone god that looks beautiful and all daemons look similar to the form of their patron god, why are Daemonettes so ugly?
It is a misnomer to believe Slaanesh is only the god of physical beauty or that anyone who worships him will be beautiful. Slananesh is a shapechanger so can be as beautiful or horrifying as she/he wants to be. As a matter of fact, his worshipers come in all sizes and shapes. Slaanesh translates from the Eldar as the "Prince of Pleasure."
Slaanesh is the God of hedonism and excess, not necessarily physical beauty, though he/she does not mind corrupting the physical beauty of others as part of his Chaotic motif.
Slaanesh has also been called the God of perfection. He wants the driven, those not content to be great in a universe of millions, but to do whatever it takes to be the best. Not just physical perfection, because he does like that, but he is also demands the artisan who seeks to create ever greater objects of beauty, the warrior seeking perfection in his mastery of weapons and death-dealing, the singer who needs one more night on stage to create their finest, greatest and most beautiful song may find themselves praying to Slaanesh, their physical appearance notwithstanding.
Slaanesh is not limited to just carnal pleasures. Those who desire to indulge in the finest culinary delights, the most beautiful artworks, even the most sensual clothing, could all be amongst Slaanesh’s disciples. It is their committment to their talents/gifts/skills that matters most. But it is to the dark side of people, Slaanesh brings to the surface. If you are willing to murder your competition to be the best, you are the kind of person Slaanesh is looking for.
Slaanesh seeks the obsessive, the compulsive, the people who are driven to the excesses of the flesh, of their craftsmanship, of their need to create. He does not care what you look like because once the touch of the Warp reaches you, you will be physically transformed. The energies of the Warp are the manifestation of dark energies of living things in our Universe; anyone who worships it or the beings who are formed from it are transformed psychically to match their inner state of mental imperfection.
Slananesh daemonettes are ugly by human standards because once they are created by the chaotic energies of the Warp and they are forever transformed by their darkest inner natures. To Slaanesh, the most corrupted and malformed daemonette is the most beautiful. They have succumbed to their need for perfection and thus fell into his grasp.
And let's not forget what they are made for: seducing the enemies of Chaos with their wiles, augmented with psychic illusions and when that fails ripping their enemies to bits with their horrifying claws and other weapons, but the victims will believe they were killed by the most beautiful specimens of humanity.
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