How to create ‘import dummy data’ option in custom wordpress plugin

I think that this would do the trick.

Edit
Of course you can leave the line starting with if( and the curly bracket after return away when you want to have it for all posts/custom post types.

/**
  * Create dummy content
  * @param [type] $content
  * @version WP4.4.2
  */
function wpse218668_default_dummy_content( $content ) {
    global $post_type;

    if( $post_type == 'CustomPostType' ) { // cpt name or post
        // Add here your dummy txt
        $content="Add here your dummy text";

    return $content;
    }
}
add_filter( 'default_content', 'wpse218668_default_dummy_content', 99 , 1 );

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