get_the_category() 返回的数组WP_Term 对象。所以

// get all categories objects
$categories = get_the_category();
// get the first category name
if ( ! empty( $categories ) ) {
    $category = $categories[0]->name;
}

$output .= \'<li><a href=" \' . get_the_permalink() . \' ">\';
$output .= get_the_title() . \'</a> \';
$output .= $category .\' \';
$output .= get_the_time(\'d M Y\') . \'</li>\';