Kuiil doesn't look much like any of the Ugnaughts seen in "Empire."
They extended the mouth-to-nose distance, made the mouth/lips more human and narrowed the nose (less pig-like) I assume to make him more empathetic/human. So he literally looks more like the love-child of Nick Nolte and an Ugnaught, which was likely the idea (someone dubbed him "Ugnolte.")
His skin coloration is also more natural human-like. The slightly unnatural skin color seen in "Empire" makes those ugnaughts seem more alien and scary to me (people forget Ugnaughts were scary - sort of little imp demons laboring in hell.)
There's enormous facial variation in the Ugnaughts seen in "Empire" which is why Kuiil works as an Ugnaught design more generally though, so he "could" have been on Bespin but unseen.