As this is a Funny Moments page, spoilers will be left unmarked. You Have Been Warned!
In the opening cutscene, Bowser tries to steal the Sticker Comet. Three Toads try to stop him, only to send him flying into it once their grip gets loosened. When Bowser crashes into the comet, he's just sent flying with a little "plink" sound effect.
When the action briefly freezes right before Bowser crashes into the comet to emphasize on the moment, Mario's eyes turn into tiny little circles.
The Toad in charge of turning the "Things" into stickers actively trying to engage Mario into shady dealings...buying more things from the wall to the left.
The green Toad at the beginning of the game suggests you help "deburrito" Decalburg to return the town to normal. Once you actually do so, he gets rolled up in it. And you can just leave him there.
Several Toads have funny statements upon being rescued, especially the guy who was wadded up and thrown in the trash.
In world 1, when you knock over an object that causes a chain reaction with the background elements, knocking everything over like a rube-goldberg contraption, is probably one of the most unexpected but humorous moments in the game.
The Sombrero Shy Guys are just hilarious to watch. On the field, they're standing there, playing a catchy tune if you stand next to them. When you enter a battle with another different enemy, though, they rush over to the enclosing screen and join the fight to empower enemies after a turn passes.
And then there's the item that Sombrero Shy Guys sometimes drop: a sombrero. How does Mario use it? By flinging it at the enemy!
And if you fight the full Mariachi band of Sombrero Shy Guys in one level they'll play a little tune...and then throw their hats at you.
If you explore Yoshi Sphinx thoroughly, you'll fight Kamek, who will turn your entire sticker collection into sandals. Even with this making it a hindrance to fight, you can still beat him with them.
He does it again in your last battle with him. Kersti's attempt at a Badass Boast is just priceless.
Kersti: We'll beat him so bad he'll think his flips are flopped!
The dried out oasis has a Toad lamenting how it used to have water. Once you fill it with water, the scene changes to what it used to be with water and plants growing. Suddenly, the Toad that was mourning his bad luck comes down floating on the water wearing sunglasses and drinking juice, using a Totally Radical accent.
Many of the Thing animations are downright hilarious to look at:
The Turkey summons a roasted turkey that attacks by breakdancing.
The Water Gun causes the game to shift into a First-Person Shooter as you shoot your enemies.
Most people know about the Goat Thing by now, but the kicker is HOW you get it in the first place. Once you hit the tree, a swing descends far away between Mario and the Goat, and Birdo appears from the top and sings as they deliver the Goat to you.
The line of Shy Guys. If you take the long way and get into the line, the Shy Guy before you is left speechless at what you're doing. If you cut in line, however, the Shy Guy will be annoyed instead.
Kamek being referred to as a hipster.
After fighting him in World 2-2, Kersti will comment "Hey, look at me, I ride a broomstick. I was into stickers before they were cool."
Waking up the sleeping Wiggler with a loud, boisterous trumpet sticker is sure to get a hearty chuckle out of you.
In each of the worlds, there's always a red Toad that needs help from being bullied by the native enemies. After saving him all the times, his house in Decalburg fills up with anecdotes of his travels. Once having read all of them, the Toad taking care of the house starts bawling in complete admiration and drops an HP-Up.
When you first run into the Boos, Kersti says "They look more "fun" than "terrifying" to me, but whatever", all while Mario is frozen in terror.
After collecting all of the Boos, Kamek casts a spell on the book containing them to create a giant Boo. As the music builds up and Mario and Kersti recoil in horror, the music stops and the scene suddenly freezes as Kamek casually walks past them. When he's gone, the music comes back and the scene continues where it left off.
In a world made of paper, the final boss is made out of cardboard.
The museum texts showing Toads treating "Things" as it they were SCPs. In their defense, those Things can be mighty dangerous! Here are some particularly good descriptions:
The Billiard Ball:
This orb would be indistinguishable from other massive spheres if not for the inexplicable number on its side. For the love of Toad, what does it MEAN?!
The Tape:
A variation of this thing has long been used by Toad security forces to restrain ne'er-do-wells. It should not be used on your little brother. I MEAN IT!
The Soda:
Apparently, whatever mad scientist created this thing thought it would be funny to load liquid into a highly pressurized container. Ha ha.
The Basin:
Plated in what is almost certainly gold, this private, solar-powered hot tub was likely overpaid for by a rich Toad who forgot that he was made of paper.
The Trophy:
This extravagant chalice looks pricey, but functionally, it's a disaster. It's too big for one Toad to drink out of, and what's that weird guy doing on top?
Kersti's snark about the door to a windmill being blocked by a windmill blade.
Kersti: What were they thinking? "Hey, guys, did we forget anything. Oh, yeah, THE FRONT DOOR ACCESS."
The entirety of Stump Glade's Snifit or Whiffit game show, from the host'spassive aggressive remarks towards Mario, to the bonus round which has Mario whacking "unpaid Snifit interns" before "they scurry back underground to the underwhelming intern lounge!" to the various questions (26 in total, not including variations) which range from simple, to tricky, to outright weird.
Snifit: My dinner last night was a delicious mushroom steak.
Snifit: This doesn't leave the room, but Princess Peach is actually a man.
Adding to this is the dialogue for entering the bonus round: "Just like last time, our lawyers advise me that I technically can't guarantee you'll live. You OK with that?"
"I agree in a legally binding way!"
"I hereby decline."
There's a book in the Enigmansion written in Ancient Stickerish. It mainly contains words related to stickers...except for the last four letters, which spell out RAWK.
The ski-lift level is probably one of the most tedious levels in the game. Naturally, Kersti's dialogue throughout the stage calls to mind the Escalator Land sequence.
Kersti's dialogue before, during and after the minecart ride in the "Bowser Snow Statue" area, especially if you compare it with her reaction to the ski-lift level right before.
Kersti: (before clearing the stage) Huh? What is this line? It must be for something exciting! Though I don't know how they can top that chairlift! Kersti: (revisiting the level) We're riding that...thing again? I'll try not to scream.
Even with what's supposed to be a Tear Jerker moment, Kersti still has one more funny thing to say before inserting herself into Mario's sticker album.
Kersti: ...and if you sell me at a shop, I'll never forgive you!
Which is also something of a Brick Joke. When you first visit the shop in Decalburg, Kersti wonders aloud what kind of price she would go for, noting that she's far too sparkly and rare to sell for cheap. The ultimate punchline? By modifying the files, you can see that she has a selling price of only 15 coins.
The Cake Thing's museum plaque warns Mario against eating it.
A dessert of such magnitude comes around once in a long while, which is why this one is under surveillance. Step away from the display, sir.
you can see that she has a selling price of only 15 coins.