Healing isn’t mitigating any damage, it is recovering from damage already sustained. This is why warrior was so bad in 2.0, it had no mitigation, it survived purely on recovery. So what did that mean for mechanics that one shot the warrior? Death, because they had nothing to mitigate the damage.

There is a case to be made however for thrill of battle. A move that grants you a +20% increase to max hp and heals you for the amount it increases your health, this will essentially always give you a 20% hp buffer no matter when you use it, even at full hp, working similarly to a mitigation tool (though not quite as effective as actual mitigation).

So no, healing is not mitigation, does that mean you shouldn’t use it? No it doesn’t. If you can provide healing that doesn’t take away from your dps (equilibrium, storm’s path, nascent flash on the MT) there is absolutely no reason you shouldn’t be using it, you’re helping keep yourself and your tank alive at no cost helping your healers conserve their mp.