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JSON formatting is a somewhat subjective matter, and related disagreements are usually best settled between colleagues.
That being said, there are some potentially valid criticisms to be made against the JSON format in the question, especially if we are trying to create a consistent, RESTful API.

The 2 pain points that stand out:

1. A map collection is represented in JSON, which isn't really JSON standard compliant, or particularly RESTful.

2. None of the pet objects have an id defined. There is a pet_id mentioned in the question, but it seems to be maintained separately from the pet object itself. If a value is accessed in the pets map in the question, a user of the API would have to manually add the pet_id to the provided pet object in order to have the id available further down the line, when the full JSON may no longer be available.

The closest things we have to guiding standards in this situation is the REST architectural style and the JSON standard.

We can start by looking at the JSON standard. Here is a quote from the JSON wiki:

JavaScript syntax defines several native data types that are not included in the JSON standard: Map, Set, Date, Error, Regular Expression, Function, Promise, and undefined.

The key takeaway here is that JSON is not meant to represent the map data type. Python dictionaries are a map implementation, so directly serializing a dictionary to JSON with the intent to represent a map-like collection goes against the intended use of JSON.

For an individual object like a pet, the JSON object is appropriate, but for collections there is one option: the JSON array. There is a usage example with the JSON array further down in this answer.

There may be edge cases where deviating from the standard makes sense, but I don't see a reason in this scenario.

There are also some shortcomings in the JSON format from a RESTful design perspective. RESTful API design is nice because it encourages one to keep things simple and consistent. It also happens to be a de facto industry standard.

In a RESTful HTTP API, this is how fetching a single pet resource should look:

Have you tried changing x to -x? this will technically "invert" the effect.