Hi there.
Json-api spec states:

Attributes may contain any valid JSON value.

Complex data structures involving JSON objects and arrays are allowed as attribute values. However, any object that constitutes or is contained in an attribute MUST NOT contain a relationships or links member, as those members are reserved by this specification for future use.

Source: https://jsonapi.org/format/#document-resource-object-attributes

However, considering this and this any nested serializer would be rendered as resource relations even though the nested structure is not a resource.