It didn't help that she died so early on - at least with Dax it comes somewhat unexpectedly, though I never saw the teaser for the season 6 finale. But why so early on, when the rest of the episode plays tricks that have no increased payoff for the viewer. Everyone knew Riker would survive.
There's more to storytelling than surprise. I think it was good that her death came early in the episode, because that meant most of the episode was about the others
dealing with the loss, which is the part that's far too often glossed over in episodic television. Aftermath is important.
There's some retroactive irony to the fact that Yar's death in the episode is essentially pointless (which is, I suppose, the point), but that that very pointlessness becomes a plot point in "Yesterday's Enterprise".
Personally I'm not a fan of the concept that every "hero" deserves a heroic death, finding it unrealistic, but seeing it brought up in this manner was nifty.
I hate what "Yesterday's Enterprise" did with Tasha's death. I think it's offensive to say her death was pointless. Armus
killing her was pointless, but Tasha gave her life in an attempt to save other lives, and it's a gross insult to every police officer, fire fighter, rescue worker, and anyone else who's ever died while fighting to save others to say that such a selfless, heroic undertaking is pointless. It doesn't matter whether she succeeded; what matters is that she
tried, and that is absolutely heroic and it infuriates me that the writers of YE dismissed it like that. "Skin of Evil" was a smart, mature approach to death and heroism because it acknowledged the realistic truth that death is often arbitrary, while YE replaced it with a cliched, melodramatic notion of a "heroic" death as somehow being a "better" way to die than any other. It was a major step down in maturity and honesty.
And look what happened afterward, according to "Redemption." Alternate Tasha didn't get a more "meaningful" or "heroic" death after all. She was captured by the Romulans, raped by one of their officers (for all that Sela spoke about a "deal" to become his "consort," it was still made under duress and thus could never be considered consensual), and then killed in a failed attempt to escape with her daughter. How is that any less "pointless" than Tasha's original death? If anything, it's far more degrading to the character. Tasha grew up on a planet terrorized by "rape gangs," was probably a rape survivor herself, and she fought hard to get away from that life and build something better -- and she ended up condemned to being a Romulan general's sex slave for the last five years of her life? It's disgusting and profoundly sexist that the writers didn't see a problem with condemning her to that fate -- especially since the only reason it was done was as an excuse to justify bringing Denise Crosby back as a new character who never really worked well anyway.