While it's true that judo can be cardio-intensive, if someone, possibly the sensei, suggests that you need additional cardio, it indicates that your once-a-week judo class might not be providing enough cardiovascular exercise for your fitness goals.
Who recommended that you do extra cardio, and what was their reasoning behind it? Are you experiencing difficulties catching your breath during judo class? While it's true that judo can be cardio-intensive, if someone, possibly the sensei, suggests that you need additional cardio, it indicates that your once-a-week judo class might not be providing enough cardiovascular exercise for your fitness goals.