I hesitate to refer to bad grammar as "idiocy," especially when talking about how a character talks or thinks or narrates. There are a lot of ways to be smart , and as someone who comes from a place where it's not uncommon to hear phrases like "your hair needs brushed," it sets my teeth on edge to see "good" English grammar held up as a virtue and "bad" English grammar treated as an embarrassing vice. I've met too many people who talk about people with bad grammar as though they're absolute dumbfucks--"why can't they figure rule X, it's a simple rule, seriously"--and it never fails to piss me off. I think people who think they're better than friends and family of mine--and than me, to an extent--because they use words or syntax that're looked down upon are assholes and have no idea how many smart people they're dismissing.

It's something of a touchy subject for me, sorry. Anyway.

In general, my recommendation is that people do what suits a story or character. If the character would use grammar that isn't standard, they should. If they're narrating the story, then the narration ought to reflect the way they speak. If it sounds natural for the character, then it's successful, and that's what matters to me.