Does Adarkar Valkyrie's ability target cards in graveyard?
Adarkar Valkyrie's ability does not target anything in the graveyard.

As mentioned in the question, the activated ability clearly targets a creature on the battlefield using the phrase "target creature". That creates a triggered ability that effectively says


  When (that creature) dies, return that card to the battlefield under your control.


The creature that the trigger condition checks is fixed as the activated ability resolves. It is not a target that you choose while putting the triggered ability on the stack.