I don't reasonably know how a person thinks at different IQ levels, and I don't think that the INT score maps over to an actual IQ score very well.
If you look at INT in 3e, 4e, and 5e (editions which use the same bonus/penalty scheme for attributes), a character with a -3 penalty to INT checks will, on average, fail tasks only 15% more often than an average person, who by virtue of being average has no bonus or penalty to that attribute. If we assume that the average IQ is 100 points, then a person who is 15% worse would have an IQ of 85. That's "low average," but it's not within the range recognized for people with certain mental disabilities.
If I were portraying a character with a low INT, I would play that character as if she had normal intelligence, and then I would deliberately make a stupid choice every so often to remind people that she's dumb.
Also, it's worth noting that Int has a couple of different facets to it. As described in the PHB, Int covers "mental acuity, information recall, and analytical skill." The Using Ability Scores section also adds "education" to that list. With those four facets, one could easily play a 5 INT character as a normal character with just a god-awful memory, or very little analytical skill.
The character might often ask questions that she should already know the answer to:
Example 1
5 INT PC: Why do you want that dusty old idol? It's not even gold.
Other PC: We were hired by the temple bring this back. It's the entire reason we came here.
5 INT PC (embarrassed): Right. I knew that. I was just . . . um . . . joking! I was joking! It's funny, right?
Example 2
5 INT PC: What's the name of this town again?
Other PC: Seriously.
5 INT PC: So I forgot. Big deal. We go so many darn places I can't keep all the names straight.
Other PC: You grew up here!
5 INT PC: Well you don''t have to be a jerk about it.
Example 3
5 INT PC: Stop him! He's running off with the, the, you know, the thing! That we need for the other thing!
The character might also simply ask someone else to explain things to her because she lacks the "analytical skill." Or maybe the character states the obvious as if she were saying something only she figured out.
Example 1
Other PC: It looks like there's guards at the front gate.
5 INT PC: Hmm. I guess we can't go that way then. We'll have to find another way in.
Example 2
Other PC: Damn, that last set of rapids put a hole in our boat.
5 INT PC: Right. Wait. So, what does that mean?