I mean, in the manga it is 100% fine because everything else matches him. It is an elaborate design decision to present him like that in fle flashback because, well, it is nanachi's POV and it makes sense that the focus of her memories would be on his slit, his whistle and his prey: the children, while everything else is blurred, darkened and damped. Using children's POV is something that is in general a pretty common trick that Tsukushi emplys, we can see that in Ozen's creepy faces or when Bon's mask gets all twisted and warped when he explains Nanachi just how blessed she is and asks to come with him after the 1st battle.
The anime, however, went with a much more blunt approach by just making him dark as fuck. I mean, come the fuck on, you can't even make out his baller scarf and its a huge part of his design's charm.>but tsukushi approved it
For all we know he could've handwaved it or was too shy to complain and nitpick. It seriously looks like they didn't look past the flashback when designing his looks. If he ends up being that way in 2nd season I am going to be displeased and let everyone know on this siberian elk-hunting bulleting board just how vivid I am.