I have a headcanon that when combined with your autistic zenitsu headcanon creates a third, cooler, headcanon!
So, I like to think that, as a demon, Nezuko has an uncanny valley undertone to how she looks. She is still very beautiful, just a tad unnerving
Now, a lot of autistic ppl aren't impacted by the uncanny valley and even find it comforting (like me, but I did check and others have the same experience)
This is part of why zenitsu thinks she is so pretty is bc the uncanny nature of her face is easier to look at than a more human face
Oh, I love this so much!
Headcanon adopted.
Also, Nezuko really appreciates the fact the he thinks she's so beautiful and likes that he doesn't really treat her any differently, and to him, she's just a pretty teenage girl rather than a demon. (I imagine it gives her a sense of normalcy.)
Also, as an autistic person, I've never really been impacted by the uncanny valley either...I've seen/heard people talk about it, but have never been able to quite see what they were apparently seeing lmao.