Preferably without implying that they are arrogant.
Joe is friendly, but he's ___, so the things he has to say aren't very interesting.
Maybe it's just me being ___, but I feel like you are making the situation more thorny than it needs to be.
I've considered words like practical, simple, and tried searching through thesauruses based on these, as well as with other words like uneducated or goofy (the latter of which probably communicates too much eccentricity and silliness to be relevant, though it gets at the extraversion in a way that some of the other terms don't). However, practical brings to mind someone that is more conscientious than average, and I would prefer a word that has minimal implications on that. Simple focuses too much on the uneducated/unintelligent part. I've also considered loudmouth, but to me it implies arrogance or unpleasantness more than it implies being uneducated.
Ideally the connotation would be neutral or even positive, as a descriptor that is as inoffensive as possible. Register doesn't matter much, but ideally it should be an adjective (though a noun is OK if no adjective can be found). It's okay for it to be a compound word, but multiple words or a phrase wouldn't work, as I need it for something where the rule is that people need to be described using single words.
The best word is would be one that contains the two meanings I described, but as little else as possible (i.e. without connotations related to other personality traits), so that it is as general as possible.