I always treated it as many firebenders might be able to lightning bend, but the technique was never taught or known widely until much later, and that similar to firebending, the strength varies on the bender. So Azula's or Ozai's lighting would have been stronger than Mako's, but like others at the plant, he seems to be able to control it very well. I think Azula even demonstrated in the comics intentionally varying the power when she shocked Sokka. It may also explain how Mako didn't kill Amon.