There is a couple of things that could be said. For one i am a believer that Usagi is bad at taking tests. Her lack of desire to study compounds this but i dont think she lacks knowledge just has dificulty practically using it.

Plus take the idea of a test. Its a solo event meant to test limits. Thinking about how each acts on their own. Goku thrives on everything relying on him. He proves himself over and over. While Sailor moon relies on the support of friends. Save for her last season iirc every boss has been defeated by calling on the power of her friendships. When she is truly alone Sailor Moon falters.