{
  "hosts": {
    "a_hostname" : {
      "ips" : [
          1,
          2,
          3
      ]
    },
    {...}
  }
}

jq -n {hosts:{}} |
for host in $( ansible -i hosts all --list-hosts ) ;
    do jq ".hosts += {$host:{}}" |
    jq ".hosts.$host += {ips:[1,2,3]}" ;
done ;

{
  "hosts": {
    "host_1": {
      "ips": [
        1,
        2,
        3
      ]
    }
  }
}