Adventure Time: Special credit goes to Kid Hero Finn who is adorable, and powerful enough to power a machine that can flip the Earth.
Knowing how badass Marceline usually is (she's an 1000 year old vampire who plays an axe-shaped bass guitar, come on!), seeing her as a little girl offering to fight in Simon and Marcy is just adorable.
Flame Princess. Can defeat legions of deadly hoards, yet you'd totally hug her if she wouldn't accidentally burn you to a crisp.
Princess Bubblegum is the sweet, polite ruler of the Candy Kingdom, who is made of sugar. She also wouldn't hesitate to rip off your limbs and literally stomp on your heart if she needed to.
Nibbler falls under this category. There's practically a whole episode devoted to how adorable the crew find him (much to Bender's chagrin), and in as the series progresses, it is revealed that the Nibblonian race was already 17 years old at the time of the big bang, and are charged with protecting the Earth from the evil Brainspawn and also that Nibbler himself had a fairly big part to play in getting Fry cryogenically frozen for 1000 years in the first place.
Nibblonians are race of Badasses Adorable. As ancient, super-intelligent beings, they resent being considered cute. However, they purr like cats and make noises like puppies. One of their leaders remarks that sometimes he fears they are indeed cute. In addition, they can kill and eat animals of any measure in one bite. Nibbler himself gets fed up with it in season six:
Leela: If you want to be treated like a fellow crew member, fine, but no more purse rides, and no more dressing you up in your cute little sailor suit.
Nibbler: I keep telling you, that's my real naval uniform.
All of the members of Chip 'n Dale Rescue Rangers except for Monty have their moments, but none more consistently than the fly Zipper, who almost never shows any fear when something thousands of times his size threatens his friends.
Ahsoka Tano has several moments in the series where she fits this trope. Considering she is around 14 in the first and second seasons, and a large number of her enemies are at least twice her size, not to mention her large eyes and small frame in comparison to the rest of the main characters, giving a slightly moe appearance, she can be cute yet badass in the course of the same episode.
R2-D2, the Badass Adorable of the original trilogy, gets to show more of his badass attitude throughout the series.
The younglings in the Young Jedi arc also count because they are younger and less sure of themselves but still jedi in training.
Tommy Pickles. He's constantly standing up to Angelica and is the bravest baby in the show.
Cowardly Lion Chuckie's very adorable with his messy red hair, freckles, and glasses, and despite his shy nature he's had his share of Badass moments as well. Like when he summons the courage to stop the wedding in the second movie. And he successfully (if a bit haphazardly) pilots Robo-Reptar with the help of the other babies. What follows is a great and crazy climax which culminates in his final fight with Jean-Claude (who is piloting Robo-Snail), and he beats him. Meek little Chuckie, who was still suffering from his mother's death, beats a fully grown man in a giant robotic snail, and makes it in time to halt the wedding.
íMucha Lucha!: Pulgita saves her brother The Flea from The Masked Toilet And She can beat everyone who came across her.
Professor Princess. A tech genius on a quest to rid the world of violent toys by blowing them up with her toy-themed weaponry. Her mount is a flying robot horse named Powdered Sugar. She has a doctorate. Professor Penny Princess is her real name.
Sari rolls with the Autobots and was actually a useful human sidekick (though she probably considered them her sidekicks). Highlights include saving Bumblebee and Prowl from space barnacle infestation and tormenting the Constructicons when they crashed the Autobots' hideout until they left. Goes double after "Transwarped."
Bumblebee carries this role, carrying over his characterization from the live-action films. He is bright yellow and partners with Raf, the youngest of the human companions; but easily plows through waves of Decepticons, being second only to Optimus Prime in body count. Even more so when in the third season finale he stabs Megatron through the Spark(!)
Ty Lee is a hot, cheerful and bubbly, well-endowed teen babe who uses her enormous...strength to beat grown men to death with her bare hands. She easily downs powerful benders, even though she has no such power, and does it while filling out a shirt like no sanely-drawn woman should.
Kyoshi Warrior Suki definitely counts. Already a Badass Normal amongst a world of supermen, she can outmatch anyone at armed combat with her fans, all the while having irresistible Puppy-Dog Eyes and the cutest pout/sassy smile ever ever.
Appa! Aang's loyal aero-bovine companion, who looks like a ten-ton stuffed toy and gets some awesome Big Damn Heroes moments.
Aang's three airbending grandchildren, who are quite capable of taking on an equalist invasion of Air Temple Island.
Korra's pet polar bear-dog Naga is the most badass mothafucka in the franchise. Bitch-slaps the Lieutenant in mid-air off a cliff, rips open steel prison cells, and knocks down gigantic platinum mechs like dominoes. She's usually quite a friendly and affectionate mutt...
The Powerpuff Girls are described on their own page as "Cute Flying Bricks".
Specifically Bubbles who is far and away the most adorable of all of them. Only one time does she ever truly get angry. After taking a level in badass she becomes more than a match for both sisters, easily beating them up by herself.
Dot Warner from Animaniacs is a cute little girl in a pink dress with a flower in her hair and she's just as devastatingly annoying as her brothers. She also carries cute little boxes that hold her.. pets.
Ding-A-Ling Wolf from Hokey Wolf cartoons. In one episode, he witnessed a robot trying to strangle Hokey and what did he do? He grabbed a massive stick and beat the living crap out of the robot. Keep in mind this is an adorable wolf cub with Puppy-Dog Eyes and an innocent personality.
Lexington from Gargoyles: He's tiny by gargoyle standards (and none too big by human standards) and has flying-squirrel-like patagia for wings, owlish eyes, and a reedy tenor voice. He's also the teamnerd. That said: he's still a gargoyle warrior. Don't piss him off.
There was a Tom and Jerry cartoon where Jerry got a bulldog about his size. Tom made the mistake of picking up said dog, tossing it up and down like a ball and giving an "Ah, how cute!" chuckle.
Danielle who looks like an adorable little tomboy and is constantly in danger due to her unstable form, proves even if she doesn't possess as much power as Danny himself, she can still dish out the damage when needed. Not many people can go and one up Vlad Plasmius.
Meap is an intergalactic cop who roams the universe busting people who do stuff they're not supposed to do. He's two feet tall, doe-eyed, pink, and all he can say without his translator is "Meap!" Meap's second appearance includes a look at his home planet, a society based on cuteness. This does not stop them from mobilizing for war. In tanks that skip.
Perry the Platypus. As his theme song states, he's a semi-aquatic egg-laying mammalof action.
Isabella has proven herself to be quite the Action Girl at times. In "Meapless in Seattle", she uses her cuteness as a weapon to beat Mitch.
The Fireside Girls as a whole. They look darling in their scout uniforms and one of their accomplishment patches is wrestling an alligator.
Phineas and Ferb. If they wanted, they could invent a Kill Sat or Humongous Mecha in their free time. They built fifty foot tree house robots to have a rock'em sock'em robot battle with their teenage sister. They used the cumulative might of their inventions to stop an invasion of killbots from another dimension! They also built a space laser when the got the blueprints mixed up for their giant sundae maker. They just left the giant laser unattached before launching it into orbit.
June appears as a cute tomboyish little girl, yet get her mad and take cover.
Loopy, though her temper isn't as fiery.
My Life as a Teenage Robot: The tiny wind-up toy robot Killgore. No one takes him seriously due to his size, but not only does he manage to capture The Hero, he also manages to rebuild a gigantic, world-destroying machine by himself.
Mort from The Penguins of Madagascar. Mort is usually rather cute and pathetic, but he has been known to beat Skipper at arm-wrestling, uproot whole trees and smash things with them, and beat up gorillas (though the last one was a result of one of Kowalski's experiments accidentally making him extremely large and muscular). Private may also qualify. Despite being very "cute and cuddly" he's been shown to be an excellent fighter, able to fend of Skipper's attacks while training blindfolded in one episode. And all without breaking a sweat.
In general, all the penguins invoke this trope to keep up their identity as zoo animals. Private is just more adorable than the others.
Any of the kids. Especially Numbuh 3/Kuki who is absolutely friggin adorable most of the time and is the team's sweet ditzyGenki Girl, but when she does get angry, epic asskicking usually ensues.
Mr. Boss: Quick! RUN LIKE LITTLE GIRLS!
Sonia. Normally she's a sweet, shy little girl who looks like she doesn't belong in a organization that fights evil adults. Then, you threaten to places her somewhere dark... and then you can only watch in horror as she beats you within an inch of your life.
Sonia: (smiles sweetly) What? I don't like the dark!
Snap appears as a cute little chalk drawing with huge eyes and a big heart, but can get very aggressive if you harm one of his friends.
Rudy is Badass Adorable as well. He's got the magic chalk which can do anything!
Kim Possible: Rufus. He's a pink rodent that can ride in someone's cargo pant's pocket and an official member of Team Possible.
Maggie on The Simpsons. She shot Mr. Burns. Also, when the whole town failed to confront the mobsters who were threatening to murder Homer Simpson, Maggie shot all of them from her bedroom when they arrived at the house. Homer didn't see her do it, but when he heard those gunshots and saw those mobsters fall to the ground, he said "I must have a guardian angel with a rifle." The irony is, this approach might have put her into some sort of Enfante TerribleAnti-Hero category if not for how obvious it was that her targets deserved it.
Bugs Bunny. How many bunny rabbits do you know who can stare down the barrel of a shotgun and casually ask "What's up, Doc?"
My Little Pony: Friendship Is Magic has loads of this to go around, what with how cute, tiny, multi-coloured ponies are living in a world full of monsters. It applies to practically all the main characters. Any badass who is a pony is automatically this. One presumes that it's inherent to Equestria's military (Consider a brightly-coloured Unicorn war hero).
Fluttershy is normally a quiet, timid pegasus who is a Friend to All Living Things, but DO NOT hurt or otherwise threaten her friends. Accomplishments include staring down a cockatrice as it was turning her to stone, taming Cerberus, and glaring down a dragon hundreds of times her size (She stood on his snout and his eyeball was larger than her). She also wrestles bears, and cowed the living embodiment of chaos and disharmony just by expressing her disappointment in him, and showing him kindness for the first time in his millennia-long life.
Rainbow Dash for rushing in at every possible threat, plus being the only pony who can break the sound barrier (Though this is unclear and even her performance is wildly inconsistent). Worth noting that she kicked the above-mentioned dragon.
Twilight Sparkle is the pony equivalent of Jean Grey, down to the huge power and even more powerful mentor. Can levitate massive weights, teleport, and reverse gravity. All of which she foregoes when a hydra threatens her friends, and she decides to charge at it instead.
Rarity, the most prim and proper of the ManeCast, has one reaction to a manticore charging at her: kick it in the face. Despite acting delicate, she's demonstrated proficiency at martial arts more than a few times. Her manipulative cunning is also not to be underestimated.
Applejack lacks the special powers of her pegasus and unicorn friends, but she can be relied on to handle a dangerous situation at least as well as any of the others. Her daily routine is kicking apple trees to dislodge the fruit; she can also kick small objects through the air hard enough to use as projectile weapons.
Pinkie Pie is usually a comic relief character who bounces along but in the big fight scene at the end of season 2, she started to harness her weirdness (and Party Cannon) towards being Crazy Awesome. Do not, repeat, NOT break a Pinkie Promise. She will hunt you to the ends of Equestria, screaming with the Voice of the Legion, to exact retribution.
Badass Adorable-ness is more or less Mickey Mouse's main characterization; he's cheerful and upbeat, but he always manages to save the day in the end. In particular, he takes a level in badass in Kingdom Hearts and Epic Mickey. He is like that in the comics, too, since the very start in 1930. Watch him fight crooks, pirates and the like with pistols, lead pipes(!) or just his fists. He might be a nice guy, but he's no sucker.
Spinelli is one of the most triumphant examples in the series, as she's a small adorable nine-year-old girl with her hair in Girlish Pigtails, yet she's the toughest kid in school.
T.J. Detweiler. Short, chubby, wide-eyed, sweet, and adorable. He's also the leader of the gang, who's responsible for all of their awesome moments.
Na´ve Newcomer Gus Griswald, who is one of the cutest kids among the main six, being the Adorkablesweetheart of the gang. He can be a bit of a fraidy cat at times though when he stands up for what he believes in, he's extremely badass.
Outside the main six, Miss Grotke is the kids' fourth grade teacher who is absolutely adorable and a martial arts expert.
Colleen from Road Rovers. She has no actual superpowers. You know those big concrete blocks that they have in parking lots to keep people from driving onto walkways or into the side of the building? She broke one of those like it was nothing, after a monster threw it at her. You know those 2 big, rabid-looking cano-mutants that used to act as Parvo's main goons? They run off in terror when she reminds them of the first time they met (she beat them up on top of a train). She also routinely beats the crap out of the doberman who has super strength and extra-sharp unbreakable claws. She also beat up a pack of werewolves, because they drooled on her and ruined her hairdo. She also turned into a large werewolf capable of piloting a helicopter.
From Titan Maximum, we have Clare the deadliest assassin in the solar system. She looks seven (Gibbs assures Jodi she's really 24) and acts the same with a love of all things cute, especially bugs and frogs. Unless she's killing someone.
Cubbi from Adventures of the Gummi Bears. Cute and badass. He dreams of being a knight and a hero, but he's proven himself worthy many times even though he's just a little bear cub. He's at his most badass when he dons a mask and takes a vigilante persona called "The Crimson Avenger". Heck, any of the Gummi Bears get a Let's Get Dangerous moment anytime they use gummiberry juice to ward off foes.
The Flintstones: Bamm-Bamm Rubble, the adopted son of Barney and Betty, could shake an entire house with his club even as a toddler. At times he even got Barney and Uncle Fred out of a pinch with his super strength.
Rigodon from Around the World with Willy Fog. He's as anthropomorphic cat in a suit and cute as a button. He's a sweet, good, friendly fellow and a loyal servant. But he can kick some serious ass. He used to be a circus artist, so he's very fit, and if anybody threatens Mr Fog, Princess Romy or his friend Tico, he goes berserk on them.
The Winx Club all six of them are cute girls who are superpowerful, and kick major ass.
Grojband has the members of the titular band themselves.