"왜 *수시를 요리할 준비되는 것 같이 보여요?" (스시 is the correct spelling for sushi).
Since "is ready" indicates a state, we usually say "준비된 것" or "준비되어 있는 것" rather than 준비되는 것.
Other than that your translation is okay, but 준비되다 sounds a bit big compared with "be ready" in English.
If someone is about to cook sushi, we'll probably use the expression -려는 것 같다 (It seems they're about to...).
So you might say: 왜 스시를 요리하려는 것처럼 보여요?