This is how i would write a stupid character. Just gonna jump right in. Ahem:
Let me tell you a few stories about how stupid my sister is.
Alright guys so there I am right. Laying in bed all comfy and warm with my adorable sister, Dolly, at my side. Now, some people would find this cute, yeah? And so would I, really, if not for the fact that at the time she was pounding the back of her head against my forearm.
"What the hell are you doing?" I'd asked.
"I'm trying to soften it up." She'd explained, complete with a frustrated grunt as she continued her attempts at tenderizing.
"That's my arm, Dolly."
"I know, it's hard as a rock." The girl complained.
And that's not even the half of it. Allow me to recount what happened the very next morning. I was sitting at the table chewing on a piece of bacon with Mother and Father when Dolly walks in with a stretch. Notices me, slides into a chair and lightly elbows my side.
"So what'd you think of them dreams last night, huh big guy?"
"...What?"
And my parents and I were all looking at each other thinking, "She thinks we all have the same dreams?
She's 12. For twelve years she thought we, me and her, had the same dreams. Like they were just programmed in, like a whole goddamn set.
"What dreams?" Mother inquired.
And the girl only passed me a knowing smile, "Let's talk after breakfast."
These stories MAY have been mostly plagiarised from Bert Kreischer and changed up a bit to suit my needs, but thats totally how i'd do it anyway. All it would take is one good blunt.
You dont have to make the characters stupid intellectually, but you can make them stupid in way of their common sense or what they think is a "good idea" Like, say, making a safe word "motherfucker" or wanting to get a wolf sleeve tattoo with the tail ending at her finger so that you know she's happy whenever she wags her finger.
Or in my case, making her think she can take over the world by using baked goods for the ultimate goal of incestifying the globe so she can legally marry her brother.