Few things bother me more than reading a story only to find that little time has passed before I think the character is a moron. While I enjoy amusing stories and even really weird ones that might make almost no sense, I am not amused by a character who is unable to make good decisions - the 'bumbling idiot' character, I suppose. I know a lot of people like this sort of thing, and no offense intended towards those who do, but it's just not my thing.
But I did grow up a little bit recently while contemplating the mistakes characters make -those mistakes that make me want to toss the book and write the character off as an idiot who can only be saved by some kind of divine intervention. While I might be able to see the right decision, that doesn't mean I should expect the character to. If my life was a book and I was the main character (a book no one would read, but work with me here) and I made one of my awesome mistakes, someone reading my story would think I was a complete moron.
So, my two cents is this: keep reading, even when a character might appear to be rather thick or maybe even hopeless. We're on the journey with them, and hopefully they'll learn from their mistakes. After all, the journey is the story, and it's up to us writers to make sure that our characters make mistakes worth learning from.