"Tell that big dumb scary face to take a hike and leave you alone and if he thinks he can scare you then he's got another thing coming and the very idea of such a thing just makes you wanna HAHAHAHA-, heh LAAAAAAAUUUGH!"
One Piece is rife with targets for this trope, constantly introducing impossibly stupid-looking monsters, only to have Luffy, as an inversion, call them really cool. Sanji tends to play the trope straight though, talking about how to cook monsters while beating them senseless.
In the second season of Slayers, Lina and company encounter a monster that looks like a floating tribal mask. Then it's body grows in and it resembles, well... a penguin wearing a tribal mask.
Lina: *laughs hysterically* Oh, you're just weird, pal.
Calvin does this to several Batman villains in "Calvin's Batman Adventure".
Jark (from "Attack of the Monsters") gets hit with this a lot as well.
Jark: I WILL DESTROY YOU! And I WILL take over the planet! MTM: Did your mummy tell you that?
In Getting Back on Your Hooves, the Big Bad assumes the form of an alicorn to terrorize the Mane Six within a dreamscape. They mock her for it, since they've fought a real alicorn and fail to be impressed.
Peter Venkman's specialty in the Ghostbusters franchise, although the others get in on the act. Interestingly, this doesn't kick in until after the first movie: after beating Gozer the Gozerian (and Cthulhu in the animated series), everything else seems a bit pathetic in comparison.
In Lacuna, Liao is less than polite to the Toralii fleet that jumps in after they release the Forerunner.
The Sword of Truth had an ancient evil appear in the form of — a chicken. That cackled. Even staunch fans of the series agree that was utterly Narmtastic.
This is the proper way to fight boggarts in Harry Potter series — use Riddikulus charm and concentrate on something funny. The boggart will become funny and powerless.
Find someone who's never heard of Doctor Who. Show him a picture of a Dalek and tell him it's the most dangerous creature in the universe. Odds are high this trope will be his instinctual reaction. Even in their own series they get mocked. "What are you going to do? Sucker me to death?" And so it did.
Even fans of Doctor Who can't take the villains of "The Invasion of Time" seriously, due to some poor handling by the writer, uninspiring costumes, and a big heap of Special Effects Failure. (Which is a pity, because their basic concept, had it not been mishandled, is frankly terrifying.) When they returned in the Expanded Universe, the Doctor's companion spent several minutes riffing on the theme of "Oh, good, it's only the Vardans!" to their faces.
On The Monkees Mickey and Mike, confronted with a mummy, berate it for being dirty and smelly. The mummy retires discomfited. (Note that despite his bravado, Mickey is actually terrified.)
In Power Rangers RPM, the Darker and Edgier series couldn't always get around the goofy monster designs of the cracktastic Engine Sentai Go-onger and had to plunge in, with even the villains pointing it out. "It looks like a bottle of window cleaner. Are you going to destroy the city or give it a streak-free shine?"
For more D&D monster mockery, visit the WTF D&D!? feature at Something Awful, as it takes the piss out of the artwork in tabletop game sourcebooks, usually focusing on ridiculous monster ideas.
Many, many PCs do this when their DM reveals a custom monster he's spent all week lovingly crafting. Not that published monsters are exempt, especially those published by 3rd party publishers.
Lampshaded in Munchkin with the Monster The DM Made Up Himself.
Many PCs when they encounter something that would be terrifying... about five levels ago.
In the Pathfinder manual, before the class chapter, we are treated with Valeros saying about ogres that they "have some uglies ahead. With faces like that, no wonder they are afraid of going out during the day."
The "Misfit Monsters Redeemed" supplement is written to avoid this trope to some degree. Basically, it takes several silly Gygax-era monsters and attempt to reimagine them as serious and digified encounters. For instance, the Flumph, originally a silly flying jellyfish prone to falling over by itself, is now a good-aligned extraplanar harbinger who seeks to warn other races of the approach of malevolent aberrations and other Eldritch Abominations. The Wolf-In-Sheep's-Clothing, which originally looked like a stump with an unconvincing rabbit-puppet sitting on it, is now a tentacled monster that uses the corpse of its last meal as a lure to draw in more victims.
Hamlet calls a ghost a "king of shreds and patches".
In the Spooky Forest, you are attacked by one of the spookiest mummies you've ever seen. [...] This isn't saying much, though, since you've never found mummies to be particularly spooky. I mean, "Ooooh, no! I'll have to walk slightly faster if I want to escape!"
Touhou 13: Ten Desires has Marisa meeting Mamizou Futatsuiwa, the Bonus Boss of the game, who she couldn't resist making fun of after finding out what she is despite it being a hazard to her health.
Marisa: But you're still a tanuki, right? The kind that drums on their bellies on the night of the full moon? Mamizou: You are making light of us, are you not? What do you call yourself? Marisa:I'm Reimu Hakurei! Mamizou:Marisa Kirisame, is it? I'll be sure to remember that. I intend to take my revenge, so you'd best watch your back on the night of the full moon. Marisa:Hey, go after the Miko, not me...
Torg and Riff from Sluggy Freelance do this pretty much instinctively. It comes in handy one time when they need to cause K'Z'K pain in order to exorcise him. He's invincible, but still suffers from their horrible puns about his name.
This trope gets its name from the review of Nezumiman where, after naming this trope, the reviewers exercise it for half of the video.
Hadriex would go on to use this trope in many future videos, getting better at it as he went. His review of the Great Giana Sisters used it to good enough effect that Blask Forest Games linked to his video back when they were making Giana Sisters Twisted Dreams.
"Where No Fan Has Gone Before": Upon seeing Energy Being Melllvar, Bender's response is "What a cheesy effect!"
"Kif Gets Knocked Up a Notch": Bender again. "What's that big, ugly thing? A frog? a toad? or your momma?"
"Benderama": The crew make a delivery to a giant alien and are warned not to remark on his hideous appearance. Natually, Bender (and the two clones of himself he created) let fly with the ugly jokes, and the giant attacks them. Later, the giant flies to Earth to apologize, but everyone is drunk because all water has been turned to alcohol (Bender's fault, of course) and ends up being insulted and going on the rampage all over again.
There is a whole song about it in part 2 of the pilot, "Elements of Harmony", in which Pinkie Pie teaches her friends to literally laugh away their fears.
In "A Dog and Pony Show", Rarity is initially intimidated by the Diamond Dogs who are holding her captive, but when they try to force her to find and dig up gemstones, she obstinately refuses, then goes on to mock their appearance, manners, and personal hygiene. By the time her friends show up to rescue her the dogs are begging for them to take her away, and pay them to do so.
Later, when Rarity gets snagged by an angry, rampaging dragon (actually a transformed Spike) she chides him for his uncivilized behavior.
The Simpsons, in a "Treehouse of Horror" episode, Homer mocks a Werewolf Flanders, while he's being mauled!
In one Static Shock episode, the Monster of the Week introduces himself as a string of gibberish ending in "ack". Static and Gear promptly go "What kind of a name is Thumbtack?"
In Soviet cartoon Two Epic Warriors Russian and Central Asian warriors ended up in wrong fairy tales, which made their enemies Wrong Genre Savvy. Russian warrior faced a giant daeva (kind of a demon) who was empowered by fear. He mocked him until he became small, then lured him into a narrow cave and said "I'm very afraid of you!" The daeva got stuck there.note Meanwhile the Central Asian hero faced an immortal undead wizard. Instead of searching for the magic needle with his death or fighting a Hopeless Boss Fight he just lassoed him and left him tied up.
This supposedly happened the first time settlers found out about the Platypus, often described as God's idea of a joke. Then we found out just how dangerous these things really are...