I get that it's meant to be a permanent transformation and honestly I love the concept of having a companion who gets really messed up by a curse, but it really looks bad. Like aesthetically unappealing in a way that I don't think it's meant to, and I know Larian can make horns and black sclera look good! Feels like they went too far and not far enough at the same time.
It also happens really early so he'll spend basically all the game like that and averting it requires you to sacrifice another companion character.
If I had to guess it's lipsync reasons, I think the dragonborn use a different rig.