Pikachu Libre definitely plays up the luchador aspect of it. In addition to wearing the obvious costume, it acts pretty much how you'd expect a wrestler to, to the point where even its Pokémon Speak sounds like it's shouting wrestling moves. However, the main thing that makes it all hilarious is the fact that it's a Pikachu acting like a macho badass wrestler would! Extra points for being a girl.
One of Gengar's victory animations has it attempting to interact with the Battle Results screen as if it's aware it exists. And "interact" means that Gengar attempts to lick the Battle Results like it were an ice cream cone.
Another has it swimming around in its shadow, diving down briefly and popping back up with a grin on its face like normal. As the screen fades to black, however, it apparently begins drowning in its own shadow for a moment.
Gengar's reaction to losing a match is to hide its face in its shadow with shame, leaving only its legs and backside visible.
The mere fact that Chandelure is playable (as well as many of the comments regarding the announcement of Chandelure's playability), thwarting many anticipated guesses such as Swampert and Heracross. One would be hard-pressed to see a chandelier/lamp going toe-to-toe (or maybe toe-to-candle?) with other fearsome creatures in other Fighting Games!
t3h_ph1r3: The day has come. I can now play as a fucking chandelier in a fighting game.
The fact that Magikarp, one of the most infamously bad Pokémon in the entire franchise, is an Assist Character in the game. And this is before it's even been revealed what it does! (Magikarp has only ever been known to learn Splash, Tackle, Flail, Bounce, and in extremely rare cases, Dragon Rage.)
We now know that it uses Bounce as a Combo Breaker. This still is hilarious, since Magikarp in this game is more effective than Goldeen is in Super Smash Bros., despite the opposite being true in all other media.
Remember Bountiful Sea, the Tekken Tag Tournament 2 fishing boat stage with many men catching various fish as they land on the deck? It's back. But this time, overflowing with Splashing Magikarp.
In the cinematic trailer, Erin's Braixen performs several flourishes with its wand before ending in a pose, wand extended and head down... Then Travis' Garchomp uses an attack to blow the wand away, causing the Braixen to huff and stomp its foot angrily.
Almost every Pokémon apes at least one win pose or otherwise iconic animation from a Tekken character. Some of these are cool, others are hilarious, intentionally or otherwise, like Garchomp mimicking Bryan Fury's infamous laughter.
While in Duel Phase, going into a high stance as Machamp causes them to strike a pose and start to sparkle.
Pikachu Libre's high stance is waving to the crowd, who then cheer her on.
A Mirror Match results in the Player 2 Pokémon receiving a recolour of red, blue, or green, as well as a stripe somewhere on their body. A Mirror Match between Pikachu results in the Player 2 Pikachu wearing blue shorts.
The weird angle on the zoom-in on Garchomp's face after it wins a round. The angle makes its face look... off, but you can't help but chuckle.
Of all the many comments you can put on your profile that have to do with the playable Pokémon, one in particular compliments Lucario's paws.
One of Braixen's intros shows it fixing its ears up and prettying its hair, with the trainer commenting "Looking good, Braixen!" Braixen's response? To reel back in shock and point at its trainer with a Tsundere attitude.
Another one of its intros has its sleeping cutely on the battlefield, until its trainer goes, "All ready, Braixen!" It quickly gets to its feet and holds a hand in front of it with a "You did not see that" glare.
One of Braixen's lose animations has it throw a tantrum where it slams the ground with whatever passes for fists on an anthropomorphic fox. It's both adorable and hilarious.
Blaziken has High Jump Kick as one of its moves, which sends down a fiery Death from Above kick attack to the opponent below. If Blaziken misses, though, it flops on its back in pain.
Croagunk's announcement brings with it several amusing moments:
The fact that Croagunk is the first Pokémon to be both playable and a Support Pokémon at once. If the player so chooses, they can have play as Croagunk while having Croagunk (and Sylveon) as a support. Right after it was revealed, one of the first official gameplay videos of Croagunk was a ditto Croagunk match with Croagunks on either side.
The various silly poses it strikes during battle, including a "draw me like one of your French girls" pose.
Croagunk's Burst Attack has it summon copies of itself that land on the opponent. It then spits poison at the pile of copies, causing the whole thing to explode. As if that wasn't enough, Croagunk salutes during the animation.
One of Decidueye's attacks is Smack Down, where he'll fire an arrow diagonally upwards. It if doesn't hit the opponent, it'll make an audible CLUNK as one of the camera-holding Magnemite drops onto the stage, potentially on the opponent's head.
The credits contain a large display of the opening and storyline cutscenes, while slowly more and more playable Pokémon show up to watch, culminating in a brief appearance by Mewtwo. The DX version plays with this. First, Darkrai briefly passes by and doesn't bother to watch. Then Croagunk reclines on top of the display, and Decidueye later joins him.
The story of DX is unchanged from the original, so it retains the plot progression of the Ferrum League. Multiple trainers will comment on a mysterious black Pokémon who none of them had seen before. Yet it is possible to get these dialogues while winning battles with Darkrai, who is equally black and legendary, but not the one they're referring to. This is particularly amusing when juxtaposed with the anime's Sinnoh league tournament, where Darkrai was the mysterious Pokémon combatant on everyone's mind.
In the Diggersby Land stage, there's a Zangoose carrying a bunch of shopping bags for its trainer (and doesn't seem too happy about it).
Mixed with Fridge...er, Heartwarming, there's a Teddiursa sitting next to a Zangoose on a park bench. In the Autumn version of the stage, that same Teddiursa has evolved into an Ursaring and is constantly trying to keep itself from leaning on Zangoose.
In Old Ferrum Town (Winter), there's a Cubchoo happily playing with the mechanical Hoenn starters that come out of the clock tower, while down below a Beartic (possibly the Cubchoo's parent) is growling at it to get down from there before they get hurt.
In Mystery Carnival, there's a Pikachu dressed as a Bedsheet Ghost chasing a Pichu in circles around a table (although the Pichu seems to be enjoying it).
This gets made even more hilarious with Mimikyu's introduction in the following generation. Even better, Mimikyu is a support Pokémon in this game (as of the DX version), so it can be brought onto the stage while this is going on!
Phos Volcano has a mining operation, and several Sableyes have taken notice, including one of them who rides around in a mine cart... and ends up on the inclined belt and unceremoniously dumped into the bin.
The Training Gym stage has a Slakoth training on a treadmill. It goes as well as you'd expect, and the poor thing is left clinging onto the handles for dear life.
: The day has come. I can now play as a fucking chandelier in a fighting game.
