A big reason why Demon Slayer works as well as it does is its villains. Sure, there are other factors, but a story is only ever as good as its hero and villain. Muzan handles the villain part of that scenario perfectly, injecting the series with a constant presence that's meant to be feared.
The fact he's always hovering in the background, be it physically or verbally, helps build his aura. He does have a few clichés tied to his character, something that can't be avoided, rarely is a character ever going to be groundbreaking in this day and age.
10 Excellent: His Appearance — he has a unique appearance that sets him apart from any other villain in Shonen
The striking resemblance to Michael Jackson can't be ignored with Muzan. It gives him such a unique appearance that sets him apart from any other villain in Shonen. It plays in perfectly with the type of character he is, making you think he'd be overly charming when in fact, he's detached from this world.
He still has a magnetism about him that lures you in, catching you off guard when his heartlessness comes out. It's a rarity to ever have a baddie look this dapper, let alone the big bad of the whole series.
9 Cliché: Overlord To His Kind — something that crops up in about any other Shonen
The villain who is an evil overlord or dictator to a group of people isn't a new one. It's something that crops up in about any other Shonen. The most obvious example being Frieza from Dragon Ball Z, who is a walking epitome of the archetype.
While it's certainly a cliché, it's one that works quite well to build up a bad guy. Whenever you have a villain's name uttered in fear or in Muzan's case, have his name lead to the death of the demon who uttered it, you instantly build them up.
8 Excellent: Use Of Multiple Forms — he's constantly using stealth to get around
One of the most interesting aspects of Muzan's is that he's constantly using stealth to get around, never staying in the same form for long. He's alternated between his typical male form to that of a female character, and eventually, a small child of an old couple.
It makes it more realistic to how he's been able to stay alive as long as he has, avoiding detection. The only other notable villain who'd used that level of stealth was Naraku from Inuyasha.
7 Cliché: Connection To Main Characters — Muzan helped turn Nezuko into a demon & set the course for the inevitable conflict
In Shonen, the main villain plays some role in the protagonist's life, normally in their backstory. With Bleach, both Aizen and Yhwach did so, helping shape who he was genetically and emotionally.
The same can be said of Nezuko in Demon Slayer, Muzan helped turn her into a demon and set the course for the inevitable conflict. As with many clichés, having the villain play a role in the protagonist's life isn't a horrible thing; it gives a reason for their final battle to happen.
6 Excellent: Secretly A Coward — he constantly cuts bait to run away at the first sign of trouble
Cowardly characters aren't a new thing to Shonen or literature as a whole. They've always existed as villains, notably with people like Starscream in Transformers. The difference here is that Muzan is the big bad, someone who's meant to be the alpha and omega in the series.
Your top tier villain is rarely cowardly. It's what makes Muzan so interesting as a character. Despite all of his ego and chest-puffing over how powerful he is, he constantly cuts bait to run away at the first sign of trouble.
5 Cliché: Desire To Be The Perfect Being — it's not a unique end goal
Muzan's end goal is becoming a perfect being, proving that his egocentric behavior was justified, that he always stood above the rest of the world, even his kind. It's not as generic as trying to destroy the entire world is, but it's not a unique end goal either.
As with many Shonen, Dragonball Z has its fingerprints all over that goal with Cell. He wanted to be the supreme being as well, possessing the same level of smugness that Muzan had as well.
4 Excellent: Preying On People's Weaknesses — he always targets people at their absolute weakest
Part of what made Muzan such a good manipulator is that he always targeted people at their absolute weakest. He'd offer them what they desired, preying on that weakness. Naturally, what they'd get in return is never what they truly wanted.
Take Tamayo, for example. She was begging for a cure for her illness so she could live on with her family. When Muzan turned her, she went berserk, murdering her entire family and leaving her alone.
3 Cliché: Willingness To Kill His Subordinates — It's a clichéd way to show him as a heartless villain who is beyond redemption
For as amazing as Muzan's destruction of most of the Lower Moons following Rui's death was, it's still a very cliché moment for a Shonen villain. Those in Muzan's archetype often are callous with their underlings, seeing them as nothing more than pawns to move around for their gain.
It's the clichéd way to show them as heartless villains who are beyond redemption. It makes the viewer instantly root against the bad guy, making it an effective cliché.
2 Excellent: Ability To Fight Multiple Foes — It makes overcoming him such a monumental feat
While it hasn't been seen in the anime as of yet, Muzan's final fight is nothing short of incredible. The sheer power he has is shown off in a way that few villains get the chance to show in Shonen.
Usually, it's always just the main character that gets to take him down in a straight one on one, that being the preferred style of fight in the genre. He manages to take on Hashira, Tanjiro, Inosuke, Zenitsu, and Kanao all at the same time. It makes overcoming him such a monumental feat.
1 Cliché: Petrifying Aura — It's an easy way to show that a character isn't to be trifled with
A major cliché with Shonen is the malicious intent of a villain being so awful that it creates an aura around them that strikes fear into their foes. It's happened in just about every Shonen series, from the Big Three to some of the newer entries.
It's an easy way to show that a character isn't to be trifled with. Demon Slayer employed it a few times with Muzan, such as when Yoriichi feared for his life during their first battle.