It looks like the main villain is sexy at first, but, later in the story, it turns out that a large portion of the hero's newly-gained allies are even more attractive - in comparison, the Big Bad is nothing special.
Or the main villain is beautiful until they go into their One-Winged Angel form.
The main villain appears to be hot, but then we see a lot of good characters introduced that are, in fact, sexier, so the trope isn't in play - but wait, all those attractive so-called 'good guys' were actually evil all along!
Emperor Evulz emits copious amounts of sparkles, and is followed everywhere by a horde of screaming Fangirls, much to his frustration, as he tries to convince everyone he is terrifying and will someday rule the world...Stop Squeeing, you nattering fools! I'm trying to scheme here! I am the architect of your doom, not some common boy band rabble for you to fawn over! ...Hey, wait a minute! That sounds a lot like Robert Pattinson in real life, doesn't it? No wonder all those Twilight fangirls give him such a hard time.
Women all over the world masturbate furiously to Hitler/Stalin yaoi.
Everyone who's ugly (& wants to be good-looking) pull a Face-Heel Turn and instantly become attractive.
A really ugly character becomes a villain and despite not actually changing appearance, is seen as sexy.
Zig Zagged: An escalatingly sexier parade of new cast members, alternating between good and evil.
Averted: Evil is, in fact, not sexy, or, at least, no more than good is.
Good in the story's plot is too 'pure' for the seductive type of characters, but the creators want to put in some Fanservice and Mr. Fanservice somehow, so they give that role to the villains.
People who were good-looking were horrible to the plain-to-homely author during his/her school years. He/she begins to equate good looks with a rotten moral foundation and abusive personality.
The author wants to show how evil can be manipulative, by making the forces of evil sexy.
Lampshaded: "Naturally, I always end up fighting against sexy people. Somehow."
"I'll make him pay for dumping me! TO VICTORIA'S SECRET!"
"Oh dear, I seem to be losing my looks. I better go brutally murder a few children."
Defied: The villain always puts her hair up and wears glasses and conservative clothing, a la Dolores Umbridge.
Discussed: The heroes have a conversation about how the villains they end up fighting always seem to be so attractive, and their frustration that their side doesn't get to have people like that fighting by their sides.
Conversed: The heroes are watching television and wondering why all the villains are sexy and scantily clad in those shows.
Evil is sexy because the world is a plutocratic Dystopia ruled by The Beautiful Elite who pay for all the expensive cosmetics and modification...using the people's money.
When you meet the villain, s/he is smoking hot...until it turns out s/he tortures you for kicks.
The presence of beautiful people doing bad things causes a backlash against the concept of beauty and the glorification of ugliness. People with good looks are rebelled against or, worse, killed.
Played for Laughs: A character starts dressing sexier following a Face-Heel Turn, but her idea of "sexy" is Lady Gaga. She shows up for battle wearing a garbage can lid for a hat, a dress made of Steak-Umms, and a pair of wellies covered in glitter glue. Hilarity Ensues.
Played for Drama:
See Deconstructed 4.
See Parodied 4. Because Alice is heavily sexualized, she eventually becomes concerned with looking sexy. In an attempt to please the villainous males more, she starts wearing a very sexy outfit that reveals and enhances most of her figure. However, she's still concerned that the outfit doesn't reveal a lot. Also, the males, heroic or villainous, all start ogling at her. She realizes this and starts wearing more, calling anybody who ogles at her a "sexist perv".