How in the world was Nappa a general in the Saiyan army given how stupid he was? He believed he could stop the krillin’s attack, but he was wrong because it could cut him in half. There’s no way he was a general given that he wasn’t smart in the battle field and off the field.
Nappa’s not stupid, he’s worse — he’s impulsive and arrogant. A SUPER important thing to remember is that ki abilities in general are extremely rare in Toriyama’s canon. Namekians, Saiyans, and Yardrat(ians?) and maybe a few others are the only ‘mortal’ species in this realm that seem to be able to use them naturally, regardless of power. Most of Freeza’s upper crust are mutants who can do things the rest of their species can’t, Earthlings shouldn’t be able to utilize that kind of power and, at that point in the series, Kienzan was Krillin’s invention and was unique to him across the entire universe (in the manga). Nappa could not have been prepared for it. Vegeta knew because he’s an extremely keen observer who spent the fight watching and tracking the Z Fighters.
People often forget that Vegeta is genuinely brilliant. He put the patterns together quickly, and fully taught himself how to read ki without a scouter just in the short time he was on Earth. He was actively learning that skill watching the fight and participating in it. Nappa kept assuming that the Earthlings were weak and fortunate, but that assumption was based on their previous attacks not doing him severe harm.
Nappa and Vegeta both operate based on their experience, and Nappa’s experience was being wildly overqualified for his work. In the exact same way that Vegeta fumbles throughout the series after Namek — Nappa isn’t stupid, he’s arrogant, and he’s arrogant because he spent his life being very good at his job.
The Z Fighters are also extreme outliers in terms of combat ability. Vegeta calls Nappa an idiot because he (like Raditz) coasts on his overwhelming strength, and doesn’t actively engage in or evolve his strategy based on any new information. It’s arrogance, from the fact that he’s very used to winning without issue, and assuming the Earthlings are just getting lucky (which Vegeta also does later in the battle).
To be fair, I also don’t think he was a general in Toriyama’s canon. He’s just an elite, which iirc is based on his power, as high intelligence was also not a very common or sought-after Saiyan trait. Being incredibly smart and intellectually dynamic is something that set Vegeta apart from his peers, and likely why Freeza knew that Vegeta would be a problem for him someday. That’s not to say Raditz and Nappa were dumb, but they are all very stubborn, and Raditz and Nappa were less eager to adapt and evolve and chase ambition than Vegeta was, since their power was usually more than enough to succeed.
tldr if nappa was stupid vegeta wouldnt have bothered trying to help him. he’s a powerful fighter who frequently gets caught up in the moment, which is why vegeta tells him to use his head. there is a brain in there somewhere when it’s not being steamed by frustration.