That’s precisely the reason he’s dumb, he grew up on a island that has no form of modernization then he grew up hidden from everything to be safe from Soohoo mom. So, yes, it is a reason he’s so clueless. He’s not going to grow up 18 years on a island then come to the city with knowledge of everything after being there for a few months.
If I follow your logic, it would mean that everyone who lives in a non-modern world grow up tone dumb. Following the same logic, everyone who grows up n that island for 28 years or more should be the same right! So why, then, is the girl smart? Like I said in my comment above, not knowing things you aren't exposed to is not being stupid. If you grew up in an island where there is no electricity, for instance, you would not know about electrical equipments and things like games and wifi- I agree. but it doesn't mean you don't have common sense. the way he is portrayed is of someone who lacks basic common sense, including things about human interaction with both males and females. He did grow up with people right?! Not the freaking jungle with animals.
The girl grew up with parents who weren’t I don’t know.. HIDING FROM A MURDEROUS PYSCHOPATH. He’s not dumb he’s just adjusting, going from one place to another is a big change especially from a nonmodernized place to a place that’s basically ran by technology. We don’t know if she left the island before, we know that HE hasn’t.
... did give him a very dumb look. I'm glad the author changed the face, but I wish she would give him more expressions than the "huh" look with the pouty lips. Σ(  ̄□ ̄||)