It’s not a comfortable question to ask, yet it is an important one. And it’s totally fair to wonder if designing a zen garden can be considered cultural appropriation if you are not a part of Zen Buddhism yourself.
First, there is no hard and fast rule for what is or is not appropriation. The most important thing you can do is be respectful of and educate yourself about the religious origins of the zen garden.
If you want to include traditional Japanese artwork in your garden design, be sure to purchase it from actual Japanese artists. Reproductions may be cheaper and easier to find but do nothing to support the cultural traditions and craftsmanship that inspired them.
Avoid using religious imagery that you don’t personally ascribe to just because it fits the aesthetic. This includes sculptures or images of Buddha. Instead, include religious or spiritual symbols, if any, that are meaningful to you.