Well, first of all you need to have an image asset of a thumb-up. Let's say you do get one and put it here: web/images/like.png
Then all you need to do is this (assuming pagination.sortable('likes', 'u.likePoints') returns the string "like"):
<th>{{ asset( "images/" ~ pagination.sortable('likes', 'u.likePoints') ~ ".png" }}</th>
Of course, you'll probably want to put the image resource in your bundle and publish it with assets:install web, for encapsulations/organizational purposes.
EDIT
There are a lot of ways to solve what you're talking about.  One would be to make a macro.
In Your\Bundle\Resources\views\Macros\bootstrap.html.twig
{% macro icon_class( type ) %}
  {% set type_class_map = {
    like: 'icon-user'
  } %}
  {{ type_class_map[type] }}
{% endmacro %}
Then, in the template you need it in
{% import "YourBundle:Macros:bootstrap.html.twig" as bootstrap %}
{% set heading = pagination.sortable('likes', 'u.likePoints') %}
<th class="{{ bootstrap.icon_class(heading) }}">{{ heading }}</th>