Yes, Yes it is.

At the end of the day, its your Effective Health points (EHP) that matters.

If you have 1000 health base, and you use rampart, that's 20% or 200 damage mitigated, meaning you had an effective HP of 1200 once you used it.
To use the same example, and a regen ticks on you just after the hit, your EHP is 1000+however much the regen healed you for, same could be said of any other heal.

An easy way to understand it is with shields, like adlo. Same example - 1000 health base, healer uses adlo for 500, your EHP is now 1500. A shield is mitigation. Same can be applied to the defense stat of your gear, or from tenacity (somewhat), or from your average parry/block rate.

In the end, your best mitigation tool is having the health to survive something - Vit is king, and so by extension a heal is mitigation, because it gives you a larger buffer than if you didn't receive that heal, it gives you more EHP than you had before you were healed.