Mirror-Cracking Ugly

Mirrors don't crack when I look in them. They scream and run away.

A common comedy trope, especially in western animation, a character's ugliness is frequently conveyed by his (or her) smiling into a mirror, and the mirror cracks as a result. A type of Informed Flaw sometimes, specially when its featured in a visual media.

Compare Rage Against the Reflection and Glass-Shattering Sound.

Examples of Mirror-Cracking Ugly include:

Advertising

This Timex commercial.

Comics

Plug from The Bash Street Kids, though chances are his ugliness will make anything break.

Film

Used in the Three Stooges short "Gypped in the Penthouse" - Larry, wondering if he really is that ugly (after stopping a clock by looking at it), asks "Mirror Mirror on the Wall, Who's the Fairest of Them All?" and it promptly shatters - "I break more mirrors that way!"
Happens with Shrek at the beginning of his first movie. Though considering as he prides himself on being disgusting and ugly, he probably takes this as a compliment.
Done earlier with Mike Myers in Austin Powers: The Spy Who Shagged Me when his smile caused his rear view mirror to crack.

Literature

The children's book The Big Ugly Monster and the Little Stone Rabbit doesn't feature any mirrors, but the monster is so ugly that his presence cracks rocks, dries up ponds and kills plant life.
Occurs with the ogres in Piers Anthony's Xanth book series.

Live Action Television

A frequently recurring gag with Herman in The Munsters.
Same in the sketch comedy You Can't Do That On Television.
Uncle Arthur. Referenced in the unfortunate film version.
In Living Color: Ugly Wanda turns this to her advantage in the Bram Stoker's Dracula spoof. Dracula has spent the skit wooing her, only to rue his success once he gets a good look at her. Desperate to leave he tells her he can't stand mirrors, and since there's one in her room, well...Ugly Wanda tells him she can take care of it. Staring into it, she recites "Mirror, mirror on the wall..." and it cracks.

Radio

In The Goon Show, Seagoon has been promoted to Lieutenant.

Seagoon: Now, let me view myself in the commissioned-ranks-only mirror.
[Shattering]
Willium: Ooh, it's never done that before, mate.
Seagoon: Well, I'll make damn sure it doesn't do it again. Take it out and shoot it!

Video Games

In Ultimate Mortal Kombat 3, Mileena's Friendship features her removing her mask and looking at her hideous face in a mirror, which then shatters (followed by Shao Kahn saying "Pretty.").

Western Animation

Used in one episode of The Ren and Stimpy Show. First, Ren's teeth are so bad that his smile causes the mirror to shatter. The next day, his teeth are so bad that smiling at the mirror causes his teeth to shatter.
Used during the musical selection "A Guy Like You" in Disney's The Hunchback of Notre Dame after the gargoyles slap a giant 1800s powder wig on Quasimodo only to find it makes him so ugly he cracks the mirror.
Played with in the SpongeBob SquarePants episode "Something Smells". Spongebob gets bad breath and is wondering why everyone is avoiding him. Patrick thinks it's because he's ugly, and suggests "the reflection test". His reflection breaks the mirror with a hammer to get away from the smell.
The witch in the Fractured Fairy Tales short "The Witch's Broom".
Gnorga, the queen of trolls in A Troll in Central Park. During her Villain Song, she passes a series of mirrors, each one cracking to a more severe degree than the last, until she torches the last one before it can crack.
Yukk! from The Plastic Man Comedy-Adventure Show is a dog so ugly he has to wear a house on his head. Not only does he break mirrors (and destroys them so quickly that he's never seen his own face), but he is so ugly that when he uncovers his face, the earth shakes and anything in direct line of sight of his face is destroyed, and anyone catching a glimpse of it screams, passes out, and/or runs away. The audience, of course, never sees his face.
Kaz in Hi Hi Puffy AmiYumi breaks a mirror by looking in it as a result of the girls trying to get him an unsuccessful makeover in "Stupid Cupids".
Played with in a Danger Mouse short. Sidekick Penfold tries to take advantage of a photo booth. The result: 'You have broken the lens. You have broken the lens.'
Lucius in Jimmy Two-Shoes.
Wade Duck in "The Ugly Duckling" U.S. Acres short on Garfield and Friends. Orson is telling Booker and Sheldon, two baby chicks, his version of "The Ugly Duckling." When confronting the Wicked Witch of the Wool and she asks "How Ugly Are You?" he answers, "I'm so ugly I need to sneak up on my mirror to shave!" (To prevent the mirror breaking as he could also stop a sundial with his face.)
Hogatha's mirror cracks when she inspects herself and sees that she has dry skin in The Smurfs episode "The Golden Smurf Award".
In an episode of The Fairly OddParents Cosmo, who had temporarily became hideous from sleep deprivation, not only cracks Timmy's mirror, he also cracks the Great Wall of China and causes a herd of antelopes far away to stop grazing near the end of the episode when his ugliness reaches its zenith.