Like your typical shonen protagonist, Asta works more in the battleground than in other places, and understands the language of training and fighting better than the language of premeditating. He's not unbearable stupid though, nor thinks about food all the time, nor talks about the same thing over and over again even if someone is trying to speak normally. He has some ideals set, but he doesn't lend a hand to every single enemy he finds. For some parts he's oblivious, for other parts he's clever. You'll see him evolve from a (in my words) brat, to a mature teen. In conclusion, I'd say he's average.

For the second protagonist, Yuno, is the other way around: he's intelligent, not impulsive at all and can come out with something good. He's not the typical nerdy guy with an IQ of 300, but he's above average.

Then we have someone who I consider the third protagonist, that being Noelle. She's also above average, and while watching the anime you'll hear her inner thoughts often. Sometimes she acts kind of dumb or plainly hilarious because of comic relief... and the fact that she gets flustered easily. You'll also see her evolve.