Get categories list with category name in custom WP_List_Table class

It is very hard to test/debug this kind of question since it is dependent upon a lot of code but it looks to me like you just need to break apart your comma separated list, get the category data, and print the names. get_categories() and wp_list_pluck() should make that pretty easy.

//Display category column 
function column_post_category($item) {
    $c = get_categories(array('include' => $item['post_category']));
    $c = wp_list_pluck($c,'name');
    $c = implode(', ',$c);
    return $c;
}