It depends on how you view things.
Some people, take "Mitigation" to mean exclusively reducing incoming damage to your HP bar (So Damage Reduction effects or Shields)
Others can take the notion that "Mitigation" means to simply reduce the effect of an incoming attack (For example, "Mitigation is anything that contributes to a Healer needing to heal you for less")
With the second interpretation, Healing can be included in mitigation.
An example would be if 3 tanks with 100,000 HP take a hit of 90,000 damage and:
- Tank A did nothing and ate the entire 90,000 damage and is now left at 10,000 HP
- Tank B used a CD that reduced incoming damage by 50%. This means he gets hit for 45,000 damage and is left at 55,000 HP
- Tank C eats the entire 90,000 damage, but then uses a CD that heals for 50% of his maximum health and so is left at 60,000 HP
In these situations, both Tank B and Tank C have effectively reduced the severity of this attack and thus can be considered to have "Mitigated" it.
The one caveat with Healing as mitigation, is that it is ineffective at preventing you from
dying outright to a particular source of damage. I.e. If in the above scenario the incoming attack did 100,000 or more damage, Tank C would die before he could self heal. (This particular caveat was a primary issue with 2.0 Warrior, who was focused more so on self healing and thus struggled in a particular fight that would one shot them)