Koi ponds are not natural environments, generally being plopped into a man made backyard environment, with no natural water access, no natural circulation, and generally inadequate food sources. So, while one can’t say that any backyard ornamental pond is maintenance-free, the degree of daily or periodic maintenance will largely depend on how your pond is designed, set up, and the choices you make for fish and plants.
From the beginning, you should design your pond to avoid common problems like too much sun exposure, excessive runoff from surrounding areas, and lack of access to electric power. When constructing, be sure to install a high quality, durable liner and make your pond large enough and deep enough (but not too deep!) to provide cool spots in summer, safe places to go dormant in winter, and hiding places to escape random predators like herons or raccoons. Plan to provide space for plenty of aquatic and marginal plants, including some floating varieties that should cover about 40% of your pond surface. Make sure you have an adequate biological filter for your pond’s volume and that your pump is strong enough to circulate your entire pond’s volume at least once (preferably twice) per hour. Aeration, particularly in the deep areas of your pond is definitely worthwhile to minimize the annual pond cleanout. Make sure you don’t overpopulate your pond with fish, and avoid choosing weedy, invasive plants. If you’ve got all these points optimized, your pond should chug along with relatively little interference. You’ll need to feed twice daily in the summer months, and potentially not at all in the wintertime, check that your equipment is operating properly about once per week, check your pond chemistry from time to time, and generally watch out for symptoms that something’s out of whack. When something does go wrong, you’ll need to be prepared to jump right in and address it immediately. Whether that all adds up to “a lot” of maintenance depends on you, of course.