Her persona off-screen (yes, her recent comments about the Wonder Woman thingy were petty at best) which also, like it or not, reflects on her on-screen portrayals. Her performance in the Thor movies is worse than forgettable, but then you might argue she had little to work with, which contradicts her comments and understanding of Sif: Marvel's Wonder Woman and how it's all about her "character arc" rather all the glitz and glam? Yea right.
Also she can easily be replaced by Alexandra Daddario or the countless other up-and-coming, beautiful brunette actresses out there and it wouldn't make a lick of a difference. In other words, mediocre.
Yes, she is pretty but she doesn't exude warmth. There is this very cold look to her which doesn't jive with Wonder Woman territory.