Can either pop this into a plugin and activate it to make the command available. Or install as a package via these instructions: https://github.com/wp-cli/wp-cli/wiki/Community-Packages

comment-generate-cli.php

<?php

if ( !defined( 'WP_CLI' ) ) return;

/**
 * Comment Generate
 */
class Comment_Generate extends WP_CLI_Command {

	/**
	 * ## OPTIONS
	 *
	 * [--limit=<num>]
	 * : Number of most-recent posts to add comments to. -1 for all. Default 5
	 *
	 * [--count=<num>]
	 * : Number of comments to add per-post. Default 1
	 *
	 * ## EXAMPLES
	 *
	 *     wp comment generate --limit=10 --count=2
	 *
	 */
	function __invoke( $args, $assoc_args ) {
		$assoc_args = wp_parse_args( $assoc_args, array(
			'limit' => 5,
			'count' => 1,
		) );
		$ppp = $assoc_args['limit'];
		$count = $assoc_args['count'];
		$posts = get_posts("posts_per_page=$ppp");
		$ids = wp_list_pluck( $posts, 'ID' );
		foreach ( $ids as $id ) {
			for ( $i=1; $i <= $count; $i++ ) {
				wp_insert_comment( array(
					'comment_author'       => $this->one_of( array( 'Mr.', 'Mrs.', 'Ms.', 'Dr.' ) ) .' Author',
					'comment_author_email' => 'Author@example.com',
					'comment_content'      => 'Lorem ipsum..',
					'comment_post_ID'      => $id,
				) );
			}
		}
	}

	function one_of( $array ) {
		shuffle( $array );
		return array_pop( $array );
	}

}

WP_CLI::add_command( 'comment generate', 'Comment_Generate' );