Neither of them looked bad, they just weren't entirely convincing as real people. They're actually 95% of the way there, but the thing about the uncanny valley is that until you're at 100%, it's just going to weird people out.
Tarkin looked better since they had to put more time into him as a character, and they also had a life cast of Cushing to work from for reference. As far as I know, they didn't have one to work from for Fisher, which is why her likeness was not quite right imo, but she only had to say one word of dialogue, so they didn't have to focus as much on her.