It's not a misconception. Her, Pinkie, and Flutters are the dumbest of the group.

>>42338859
That's definitely an underappreciated one. People in general tend to underestimate Applejack's intelligence. They see 'southern hick' and automatically assume it means she's stupid. I think she's actually the second smartest in the group behind Twilight, she just never really gets many opportunities to show it off. But out of all of them, I think she's the one that would probably be the most reliable in terms of good advice on how to handle a situation.