I think they're of average intelligence. In watching their older vlogs, you could tell they were more "real" about their personalities. As they started doing more travel-type places, i.e. museums, cultural sites, aquariums, theme parks, etc., Tim asserted his "knowledge" a bit more and if he didn't know something, he'd say it and ask (seemed more genuine).
It seems like in the last 2-3 years or so, they went full-on dumb dumb where they just go full throttle stupid and ask the stans so many idiotic questions that I really feel they know the answer to 99% of the time. I think someone else said it earlier, but they do this because they figured out it works to their benefit with the algorithm. Every now and then, Tim will still assert himself as the "expert" (and often be wrong) BUT a lot of this is staged, IMHO.
Jenn is what you'd call average intelligence. However, she's chosen to master manipulation over anything else and part of that is the helpless act she portrays not just for the camera, but for Tim. She has to keep him feeling like he's the man otherwise her meal ticket might realize what a bitch she is and leave.
Tim is smart in his own way. He has a curiosity for how things work and he gets into the details, probably a little too far for his own good. He doesn't get overwhelmed because to do that you have to be able to see past the present. He's calm, that's his charm and appeal. For every day stuff though he's quite below average intelligence and his ego gets in his way otherwise he'd see that and not open his mouth. It's not an act on his part.
Tim was conned into his relationship with Jenn thinking she loves him when she only tolerates him as a servant. He jumped on the McManson, the Gucci Sweater and Tesla without understanding how he was impacting their future. Man, he even posted a ton of "Hot Chick does ..." clickbait titles featuring Jenn. None of those make the family look respectable (and they'd be smart to take them down before Jackson gets to school age).
Intelligence wise where 0 is no brain function and 10 is the highest level of genius (Most of us are around a 5 or 6)
I'd give Jenn a 5 out of 10.
Tim gets a 4 out of 10.
To compare to other vloggers:
Cory gets a 5 out of 10 (he might be a 6, he's usually vlogging after a long day of work)
Dev gets a 4 out of 10
Josh gets a 7 out of 10 (trust me, I didn't want to rate him this high, but he has a low EQ)
Jenna is about a 5 out of 10.
It's harder to rate those who have a team of good editors so I won't.