I don't think he's overconfident at all. He just never had a worthy oponent in the story so far.
It would be like calling Sukuna overconfident when all his oponents so far were tiers below him.
Funny how I read things differently (but this is why forums are made for I guess).
Both fought oponents who were tiers below them, but strangely Sukuna showed more empathy towards them than Gojo.
-Against Jogo: he acknowledged his strenght before finishing him.
-Same against Ryu.
-And in a way same for Megumi.
The only one he's been loling at was/is Yuji. Otherwise he seems to respect strong oponents (even if tiers below his own), what Gojo never did.
What I'm saying must seem strange I agree, but in a way Sukuna sometimes looks like more humble and "human" than Gojo.
(Sorry, I have hard times translating my ideas, I'm a bit tired today tbh... ^^ I'll try to put it more accurately later if required)