She is a bad actress... Or at the very least she's terribly miscast in this role. The whole show is a mess so there's plenty to complain about, but I think in a show where you do see some very good actors, the bad acting is especially noticeable. I think some people are frustrated because as soon as you level some fair criticism, the race card gets thrown out and then everyone starts praising the actress as being amazing when really she's pretty awful.
But Star Wars has rarely been known for great acting. It's better known for having very charismatic characters like Han Solo or Lando, or many others. This Third Sister character lacks charisma, even for a villain. And there's no interesting texture like you get with Darth Vader, Palpatine, Darth Maul, Dooku, or even some of the Imperial officers. It's just a moody chick awkwardly saying lines.
But Hayden Christensen is still the worst Star Wars actor.