TL;DR: Mortal Strike (and similar effects) are not made any more or less desirable by Dampening

Mortal Strike (and similar effects) are applied multiplicatively. That means they don't become any more (or less) effective as Dampening begins to stack.

Here's a quick, if mathy, example: Say you cast a heal that, unmodified, would heal for 100k. If Mortal Strike is active, that heal is reduced by 25%,

wow items

 for a final amount of 75k (and a loss of 25k healing).
Now, say Dampening is at 10%. Your 100k heal would then land for 90k. Mortal Strike would only affect the remaining 90k healing, putting the final amount healed at 67.5k. The heal was still 25% less effective than it would have been if Mortal Strike wasn't active, but was only reduced by 22.5K -- which, you may notice,

buy wow items

 is 10% less than the previous example, keeping it in line with the effects of Dampening.

 

TL;DR: Mortal Strike (and similar effects) are not made any more or less desirable by Dampening.