The Kawasaki Ninja 400 is a sporty motorcycle that is popular among riders of various heights. While it is true that at 6 feet tall, you may find the Ninja 400 to be slightly smaller than ideal, it can still be comfortable for many riders of your height. The bike has a relatively low seat height, which can make it more accessible for riders with varying heights.
It's recommended that you visit a Kawasaki dealership or a motorcycle rental service that offers the Ninja 400 and sit on the bike to assess your comfort. You can also take it for a test ride to get a better feel for how it handles and if it suits your riding needs.
Keep in mind that there are also various aftermarket modifications available, such as adjustable rearsets or seat cushions, that can help tailor the bike to your preferences and increase comfort.