This formula can work, as demonstrated by one punch man. I think it's just that the stakes are currently to low and the reader isn't invested enough yet to have a dragged out arc like this. They should have probably waited untill chapter 10 or so to start this arc.
i don't recall Saitama being
this antithetical to operating in a society, though. He has his moments, sure, but he knows he's strong and has some amount of situational awareness when dealing with a crisis or with others.
This takes the gimmick and turns it up to 12, seemingly.
And while you could be correct about the author simply jumping the gun on when they introduced this arc, the degree of Kinoko's "issues" mean that, for them to persist and carry the gag, everyone around her will have to keep secrets - from her and from the world at large, and will have to twist their words and actions in favor of maintaining her idiocy, even if doing so would countermand their usual behavior.
Because look at how many witnesses there were to this. And so when Usoyama's uncle shows up, do they just capture everyone, and sequester
all information from the rest of the government and the public at large?
Usoyama wanted to keep this a secret so she could milk Kinoko for training and advice. Doing that has nearly gotten her killed. Is she going to decide others need to know, because Kinoko's obliviousness is a clear danger to those around her? And I ask the same of the sister, once she's made aware of what transpired.
And the more people who find out and bend themselves around perpetuating this gimmick, the harder it will be to care about any of the proposed stakes that we've been presented, including Kinoko's own stated goal of overcoming her social anxiety.