I think it may be saturation to him.
It might also be tied to balancing issues , I myself don't like how matchs go against Joker :
Neutral --- somebody win neutral and do their stuff --- Joker is at 70% and he get Arsene / he did a 40% string on you --- If he has Arsene , you need to play hit and run for 30 seconds.
Here's the problem : In this game , 70% is still a middle ground for a lot of characters. Only some heavyweight and glass canon can consider this as "Death %". You still need to commit to a pretty laggy move , like Bowser forward Smash , or maybe Metwo's disable (down-b) to slightly charged D-smash.
And trying to challenge Arsene is like trying to drink milk that stayed in your fridge for months : It's not a good idea , and it's not worth it.
If you win an interaction , cool ! If he win one , he's going to hit HARD. Not counting the fact that his F-smash is hard to punish on shield and kill at 80% at the middle of the stage with Arsene.
And even without Arsene , he's amazing at gimping and edguarding , so he's still dangerous.
But if he can't achieve something with Arsene... He's pretty much screwed for this stock.
So yeah , while I think the way he's balanced is a bit unpolished with some moves , he still has weaknesses. It's just that playing a run and hit gameplay for 30 seconds in every stock is plain' ol' boring , and that him winning 2 interactions + random F-smash will get him a stock when you destroyed him for a while can feel frustrating. And the Ultimate meta is actually slowly favoring the "You're going to approach first !" mentality... Only Pichu can break this mentality well , because his frame data is insane.
So yeah , + adding another counter as a special move is... Boring. Special moves are what define the character's personnality and concept. So when they are shared between differents charcters , even with tweaks , it feel less unique.
And yeah , the anime esthetic can seem a bit oversaturated , even if him and Chrom are the only one like that in Ultimate. But then I'm supporting Camilla , Henry , Billy Hatcher , Loki and Zizou Olympia , so I can like more manga-ish characters , or more shonen-ish characters.
I personnaly don't like anything about Joker myself , when I liked Cloud and others manga boi in Sm4sh DLC , so I can understand him.
Edit :
BTW , Erdrick isn't confirmed. A SE rep isn't confirmed either. And we don't know if the leaks are legitimate or not. I still don't know why people listened to Tansut once and didn't tried to dig deeper... Always double check everything , it's the internet.
I know you didn't said "Erdrick is in !" , but the way you worded it implied it.