A cleanout is unnecessary and just adds to the overall cost.
The bottom drain suction line will be carrying debris to your filtration equipment. The most likely spot for debris to be caught is when a pipe reduces from one size to another. Having a clean-out at that point allows you to remove any debris that is caught, should that happen.
In the construction of your koi pond, you will have at least one three inch bottom drain suction line that reduces to two inches in order to connect to your filtration. When that reduction in size is made, it is imperative that you install a clean-out to help with the transition.
Place clean-outs above ground near the filtration, in an inconspicuous place that’s easily serviced. Install one on each bottom drain line that you have, prior to any pipe size reductions and manifolds you may have. It’s best to install in a horizontal orientation.