How can I straighten bent refrigerator radiator fins?
Your efficiency probably isn't harmed too much by this—you still have basically the same area of radiator fins. If you had a fan blowing through the fins (e.g. on a computer heatsink) I might be a little more concerned because active airflow is an important part of the cooling. But how much active air circulation is really going on behind your fridge?

Anyway, if it's bothering you, you can buy a "radiator fin comb" or "fin straightener", which is basically a tough plastic comb that you can use to straighten the fins. They make them in different sizes based on the spacing of the fins, so get a multi-size comb or measure your fins first.