OK, we'll see, post by post, over a long period of time, how often Joey is portrayed as not being quite on top of things intellectually. The first example I have found in a systematic search is from 102. Here, Joey loses track of an analogy that the others understand.:
Monica: What you guys don't understand is, for us, kissing is as important as any part of it.
Joey: Yeah, right!.......Y'serious?
Phoebe: Oh, yeah!
Rachel: Everything you need to know is in that first kiss.
Monica: Absolutely.
Chandler: Yeah, I think for us, kissing is pretty much like an opening act, y'know? I mean it's like the stand-up comedian you have to sit through before Pink Floyd comes out.
Ross: Yeah, and-and it's not that we don't like the comedian, it's that-that... that's not why we bought the ticket.
Chandler: The problem is, though, after the concert's over, no matter how great the show was, you girls are always looking for the comedian again, y'know? I mean, we're in the car, we're fighting traffic... basically just trying to stay awake.
Rachel: Yeah, well, word of advice: Bring back the comedian. Otherwise next time you're gonna find yourself sitting at home, listening to that album alone.
Joey: (pause)....Are we still talking about sex?<<<
Here's another one, from 104. Joey has a syntax problem.
Scene: Central Perk, everyone is there
Joey: Probably kill myself!
Monica: ..Excuse me?
Joey: Hey, if Little Joey's dead, then I got no reason to live!
Ross: Joey, uh- OMnipotent.
Joey: You are? Ross, I'm sorry.. <<<
Later, in the same episode, Ross has been continually whining about how everything reminds him of Carol. Chandler wants him to get his mind off it, but Joey does not understand.
Ross: Peach pit.
Chandler: Yes, Bunny?
Ross: (points) Peach pit. That night we, uh- we had-
Joey: -Peaches?
Ross: Actually, nectarines, but basically...
Chandler: (to Joey) Could've been a peach.
Ross: Then, uh, then we got dressed, and I-I... I walked her to the- (looks up, realises, and points) -the bus stop... I'm fine.
Joey: Hey, that woman's got an ass like Carol's! (They turn to stare at him.) What? Thought we were trying to find stuff. <<<
And here, from 109, something that indicates a career actor does not know how to apply stage makeup appropriately:
Joey: Hey, hey.
Chandler: Hey.
Phoebe: Hey.
Chandler: And this from the cry-for-help department. Are you wearing makeup?
Joey: Yes, I am. As of today, I am officially Joey Tribbiani, actor slash model.
Chandler: That's so funny, 'cause I was thinking you look more like Joey Tribbiani, man slash woman.<<<
Later in the same episode Joey ends up as the poster boy for VD. He was not informed enough to realize what he'd signed up for.
More later. I have to go. This thread is going to go on for awhile.
It isn't that I particularly dislike Joey or want to knock him; it's just that the preponderance of evidence says he is slow on the uptake at times, less well educated than most and more inclined to come across as stupid than the others on any given day. He most certainly does have intelligent episodes and the others have stupid ones, but Joey-as-idiot is a common comic crutch.