I could've sworn I'd already done a post on this. I must've included it in enough book reviews that I felt like I'd blogged about it already. This phenomenon is why the last book I threw against a wall got that particular treatment.
Too stupid to live is so prevalent it actually has its own acronym, at least in writing circles. I've seen TSTL tossed out as blithely as tl;dr, and accepted just as readily. If one of my friends tags a book as TSTL-containing, I'll steer far clear of it.
What defines the TSTL is that the plot wouldn't work without this character's total lack of common sense. If she (and let's be honest, most TSTLs are female) thought for two seconds about how reckless an action she was making, she'd do something with a higher survival rate, and there would be no conflict.
That's not to say I expect all characters inhabiting a novel to be intelligent and observant and able to kick your butt at chess. I like reading flawed characters. It's just that the TSTL stretches my disbelief too far. If I can't imagine the character surviving to adulthood outside a padded, reinforced bubble, I'm not apt to believe their existence, even with whatever world you've set up around them.
You can certainly have characters with blind spots, and whose weaknesses are exploited within the narrative. That's how stories are supposed to work. But, if the character flaw can be boiled down to, "This character would be dead several times over if not saved from the character's own bad choices," please, rethink your characterization.