How To Bulk Import wp_postmeta records in an API call?

We can use the existing meta property (see docs) to add multiple meta fields (in a JSON format) when we create a post via POST method:

POST: example.com/wp-json/wp/v2/posts/

Body payload:

{
    "title": "My title",
    "content": "My content",
    "meta": {"myfield1":"myvalue1", "myfield2":"myvalue2"}
}

or when we want to update a post with ID 123:

POST: example.com/wp-json/wp/v2/posts/123

Body payload:

{
    "title": "My title 2",
    "content": "My content 2",
    "meta": {"myfield1":"myvalue11", "myfield2":"myvalue22"}
}

Note that we will have to register the meta fields with corresponding schema properties (see docs and docs), for example registering meta field myfield1 as a string with:

add_action( 'rest_api_init', function() {
    register_post_meta(
         'post', // post type
         'myfield1', // meta key
         array(
             'single'       => true,
             'type'         => 'string',
             'show_in_rest' => array(
                 'schema' => array(
                     'type'    => 'string',
                     'default' => '',
                 ),
             ),
         )
     );
} );

We can see how the meta scheme property is checked in the rest posts controller:

if ( ! empty( $schema['properties']['meta'] ) && isset( $request['meta'] ) ) {
    $meta_update = $this->meta->update_value( $request['meta'], $post_id );

    if ( is_wp_error( $meta_update ) ) {
        return $meta_update;
    }
}

https://developer.wordpress.org/reference/classes/wp_rest_posts_controller/create_item/