It really shouldn't be, though. I.e. not unless you're parsing or emitting it ad-hoc, for example by assuming that an expression like:

  "{" + $someKey + ":" + $someValue + "}"

produces a valid JSON. It does - sometimes - and then it's indeed easier to work with. It'll also blow up in your face. Using JSON the right way - via a proper parser and serializer - should be identical to using YAML or any other equivalent format.

Even if the APIs for both were equally simple, modules for manipulating json are way more likely to be available in the stdlib of whatever language you’re using.

If you are using any kind of type checking instead of blindly trusting generated json it's exactly the same amount of work.