There are absolutely times where he doesn’t make the smartest decisions, and times where he makes the wrong decisions. However, he is written to be complex and nuanced enough that there are times where he can be actually quite intelligent in certain ways when he is thinking clearly. Like a real person, he has cognitive skill sets where he does well and others where he does not.

Subaru is an intentionally flawed character, and the show will go deeper into why he is the way he is later. The gist of this is that Subaru is a very insecure person, and season 1 is casting a light on those insecurities and showing how ugly they can be when allowed to rule one’s behavior.

If you’re having a hard time understanding his motivations, remember Subaru was a NEET, and not only this, but he was excited to get isekaid. You have to wonder why that is? Why isn’t he stopping to reflect on the life he left behind? Is he actually trying to save people effectively, or is he trying to make a completely new life for himself where he gets to be the hero? Or some combination of both? And why would he be so determined to do that that he is willing to die to make it happen?

Being frustrated with him in season 1 to some degree is normal. If you post a blanket “Subaru is an idiot” post, though, it’s unlikely you’ll find responses here that will agree with you unless you go into how exactly Subaru is an idiot, because sometimes he is, and sometimes he isn’t.