In the opening cutscene, Mario gets taken by surprise by the portal to Lake Lapcat opening up below his feet. After trying to stay aloft for a while, he pulls an interesting twist on the Gravity Is a Harsh Mistress trope: he looks at the fourth wall instead, as if he knows he's screwed, right before the portal swallows him up.
In response to Bowser Jr.'s request for Mario to beat his dad and turn him back to normal, as well as all throughout their talks thereafter, Mario looks the most amusingly skeptical and impatient he's possibly ever been in this kind of situation, not even pretending to give Junior the benefit of the doubt. Mind, this reaction is perfectly justified considering it's Bowser and Junior we're talking about here, but it's the funniest in particular that it's Mario who's not giving Junior an inch, here.
In the ending, the paintings reveal that the reason Bowser became Fury Bowser was because Bowser Jr painted his dad black while he was sleeping. Yes, he angered Bowser to the point it turned him into the largest and most terrifying transformation he has ever had... and it turns out to be mostly cosmetic and Disproportionate Retribution.
The "Surprise Prizefight" Shine is one of several that locks you in an arena of enemies...but when you defeat the enemies, no Cat Shine spawns. Turns out the game is playing a joke on you, because that wasn't all of the enemies — there's an invisible Coin Coffer hidden between two platforms whose defeat clears the mission.
The ending sees Mario, Plessie, and Bowser Jr. beat Bowser by transforming into Giga Cat Mario and Giga Cat Plessie and belly flop on top of him, complete with the rushing of air and choir singing prior to impact. With three Giga Bells Mario and Plessie are now so gigantic they make Giant Bowser look tiny in comparison. The cherry on top is how Giga Cat Plessie looks just as goofy and cheery as regular Plessie, rather than becoming more intimidating like Bowser's forms or even Mario's half-serious one. The result is of a childish cartoon kaiju annihilating a badass intimidating one that's suddenly the much smaller player, perfectly exemplifying the ridiculous tone Mario loves to use. And if that wasn't enough, you can see Giga Cat Mario doing one of his trademark poses with his happy face on.
Bowser's back to normal, but he's still gigantic and tries to go after Mario even as Bowser Jr. tries to hold him back, all while Mario stands there cross-armed and unimpressed, fully embodying the cat vibe of his suit. After he gets his dad to calm down, Junior approaches Mario as if he was going to thank him for his help… only to just roar in his face and fly away, leaving Mario only to shrug.
The fact that every creature you see has cat ears. Even the background wildlife, such as the seagulls, which is makes for a clever Japanese Visual Pun since the word for seagull, 海猫 (umineko), is derived from the kanji for "sea" and "cat".
he's possibly ever been in this kind of situation, not even pretending to give Junior the benefit of the doubt. Mind, this reaction is perfectly justified considering it's Bowser and Junior we're talking about here, but it's the funniest in particular that it's Mario who's not giving Junior an inch, here.