Episode 32; seeing all of the Cures smiling with Dull Eyes of Unhappiness is pretty unsettling. The dissonant music doesn't help.
That was actually a scene lifted from Yes! Pretty Cure 5; fitting, as Joker is an Expy of Kawarino, the one who brought that scene with the similar 'Nightmare Mask'. However, Joker gives a more nightmarish continuation and conclusion: while Nozomi brought her friends back to their senses with mere words, Miyuki is forced to fight an Akanbe in a losing battle alone while her friends just watch with the same expression, and every attempt of her calling out for them to wake up just fell to deaf ears. That's like you're in a nightmare and none of your screams of help do good to get you out of it, even if helpers and friends are nearby. Good thing Miyuki is a Plucky Girl and her refusal to stand down eventually broke the spell.
When Wolfrun gets EXTREMELY mad, he sure does makes a ferocious appearance. Sporting sharper fangs and claws, along with red glowing blood-lusted eyes.
In Episode 40, the fact he was threatened that he was going to kill Cure Happy after beating down Cure Sunny made it even more unsettling. Episode 44 shows that he REALLY wanted Happy dead.
In Episode 42, Majorina kidnaps two of Cure March's siblings and using them as human shields while she brutally beats down their older sister. After March getting her Heroic Second Wind, Majorina goes full revenge mode and outright fires an energy beam at her remaining siblings, which seemingly kills them. March's face of horror and shriek of terror combined with Majorina's sadistic laugh over her loss made the entire scene really dark. Thankfully, all of the Midorikawa siblings were saved by the other Smile Cures.
Many consider Joker to be this himself. Especially some of his expressions.
The faces Cure Happy◊ and Cure Peace◊ make during the battle against him in Episode 22 don't make things any more pleasant either.
In episode 40, he summons Wolfrun and starts to berate him for failures, his back to him the whole while. Wolfrun gets angry and defensive...until Joker finally turns around to reveal a mouth full of fangs and glowing red eyes far wider than he's ever made them before. And then he makes this expression twice more in the next two episodes. Meep.
When he threatens Majorina specifically, he makes the skull of an animal emerge from her cauldron, and the shape of it makes it seem like it's Wolfrun's. Yikes. Though he turns out to be alive 2 episodes later.
Episode 43; while Joker breaks Reika by talking and mentions her two selves—Cure Beauty vs Reika, he shows her a giant mirror. Beauty looks into it, seeing her reflection...and Joker briefly takes control of it, so now her reflection has his creepy Slasher Smile and wide eyes. And talks in his voice.