YAML's way, way more complicated that JSON. To pick one rough measure, the spec is about 7x as long. It includes a bunch of things that could be seen as negatives, such as nine(!) different ways to write multi-line strings (https://stackoverflow.com/questions/3790454/how-do-i-break-a...).

I don't have a single competitor to recommend, but JSON5, TOML, JSONC (mentioned in a sister comment), or something else along those lines might be better. I'd probably just go with whichever of those is popular in your community.