I wonder if it's an unwanted side-effect of the author exaggerating Nezuko's body language/attitude because she can't talk? (and also for comedic effect)
or maybe the author didn't have a good feeling of the character at first and has decided to change things a bit so she'd feel more like a little sister...?
5 is too much but it's true she feels more immature in this last chap (56) but in Zenitsu's dream, she seemed normal ╮(⊙▽⊙)╭ ??
I think the author wanted to emphasize the fact she's no longer human, as well as not a 'typical' Oni. If she was an Oni that could talk normally, that would take a lot of the emphasis away from her, as all the other Oni that we've encountered so far could converse perfectly normally. And it creates more mystery surrounding her, which works perfectly with how she's just so... unnaturally strong compared to the other Oni, with possible new abilities and powers popping up here and there. I don't think it has much to do with portraying her as an immature little sister or of a regressed mental age.
I hadn't picked up on that but you're right!
I did notice she's strangely strong. I wonder if the villain's blood (i forgot his/her name) is very compatible with her? Because it seemed like when he/she injects it in some people, they get deformed and die, like a rejection.
Thank you for sharing your observations!
Did Nezuko regress in her mental age? Before she turned into an Oni, she seemed more "mature" but now it's like she's a 5 year old kid