Street smart but not book smart. I have a few characters like this.
One is named Malaya Tapang. She's an incredibly strong, half-blooded phoenix. However, she's a little air-headed, bold, and expressive. One of the smarter characters of the series says something along the lines of, "Well, Malaya doesn't take time to think, so I can only assume she would rush in like an idiot and get herself killed." and Malaya has no answer because... well, he's correct. However, at the same time, the same smart character whole heartedly believes that Malaya will be who beats the antagonist. A lot of the characters are a helping hand that stop Malaya from getting herself killed, like jumping into a fight too early. You could always add a character like that, that warns the air-head, "Hey, maybe this isn't the brightest idea."
Another one I have is Rowan Mayle. This is a more modernized world. He's a business consultant and incredibly smart in multiple different fields, however... he doesn't even know what medicine to take for a headache. This is because he grew up wealthy and pampered, and if he had a headache, someone got it for him. Little details that can make your character stupid yet smart.
The other main character of this series is Sage Gannon, an intelligent doctor who is known as a prodigy. Yet Rowan has to explain to him that the city is not always what it seems, and there is crime lurking about. This is because Sage is a workaholic and heads from the hospital to his place instead of being a person, so he's completely unaware of the violence that occurs in the city sometimes.
As long as there is a reason why your character can be a bit of a dunce, there should be no problem.