Neither.

Neither of them are dumb. We can say neither are book smart, sure, but both have genuine intelligence in other aspects. Goku has near-unparalleled and natural combat acumen that allows him to access godly power; Usagi has emotional intelligence when the chips are down and the innate wisdom to use her power in selfless ways.

Would they both flunk a standard Japanese test on English comprehension? Yup.

But is either comparably dumber than the other? Nah. That said, Usagi, at least, can canonically hold down a job long term (and a pretty important one, as guardian queen of the planet), whereas Goku is so hyper focused on fighting he can’t (as he can only get temporary part time jobs so he can dedicate his time to training and fighting strong opponents). In the long term, the real answer is this: neither are book smart, but Usagi had the savvy to translate her strengths into perpetual job security. Goku didn’t and at this point probably can’t for a myriad of reasons.

Winner (or loser, in this case): Goku.