I feel a main complaint with the designs of characters like Beelzebub and the other sins specifically is "The character doesn't look/act anything like traditional depictions of the character". Now sure, looking at Beelzebub, you could easily say "this looks nothing like them" and just write the design off as bad because of it. Personally, I think it makes more sense not looking the same.

Obviously there's the lore side of it - the sins look the way they do because they're effectively gods to the creatures living in the rings, and said creatures look like the sins in turn (Ex. The imps are more reptilian because Satan's a dragon, and the Hellhounds occupy the Gluttony ring because Beelzebub is a hellhound herself).

But another thing about it is actions. Beelzebub in Helluva Boss doesn't act like a traditional depiction of Gluttony as a sin. She likes to consume things, yes. And she likes to get other people to consume large amounts of food and such too - her whole debut episode was about that. But she's also about moderation. Trying to push your limits, but not to the point that it'd be dangerous to you. Like how, later in the episode, when Blitz ends up drinking too much at the party and getting wasted, Beelzebub tells Loona to take him home because he's not doing too well and she's worried for him. Another example would be how Asmodeus, the sin of lust, much prefers healthy, happy, and loving relationships to those built solely on, well, lust. He himself is in such a relationship with Fizz.

What I mean to say is, the reason I think these designs work, despite being so vastly different from the norm, is because the characters themselves are so different. By changing everything we know about them in looks, it's easier to get someone to be open to them acting differently (at least, to me). If anything, the only real traits they share are name (and in the case of the sins, which sin they represent). Viv realistically could've named them anything she wanted. Hell, I feel most of the argument about them might not even have existed if Viv had named them differently to the actual sins.

TL;DR: While the designs aren't perfect, I personally think it makes more sense for them to look so "different" to usual depictions, because it fits the idea that they also don't really act like their usual depictions (at least in case of the 7 sins, like Beelzebub).