From a literal definition viewpoint, mitigation just means lessening or decreasing the effect of something, usually with the connotation of the thing being lessened being negative or harmful. In that sense healing could be seen as mitigation against the effects of the harm or damage done. It really depend on what you are conceptually trying to mitigate, the harmful thing itself or the harmful effects of the thing, where trying to mitigate the thing itself would be framed as preventative while trying to mitigate the effects would be framed as being a reaction to the thing already occurring.

However, in MMO lingo mitigation tends to refer specifically to preventative measures to lessen or decrease incoming damage. Healing is done after the damage is done to recoup the loss of health, and thus is not preventative and is often classified as being "sustain". So within the context of an MMO, it is best to stick to this commonly accepted classification.