As much as I'd like to just go out on a limb and say that she can't act at all, I don't want to do that just yet.
I've only seen three movies she's been in, two of them being the Transformers films. (The other is Jonah Hex, if you wanted to know.)
Now, action movies in general don't really consist of good acting, because it isn't required. She fulfilled her job as the love interest and eye-candy for the audience, and therefore did the job fine. Tell me, really, would you go into either of those films thinking "hey, we might see an Oscar-winning performance today"? No. And if you were, you don't know the subject matter we're dealing with here very well.
Once I see her in a couple of dramas, then I'll decide. Passion Play, even though it has gotten terrible reviews, interests me strictly because of the cast. I will see it, and that's likely the best chance Fox has actually gotten to truly act.
Oh, and I think that acting alongside CGI will always be really difficult, so the all of the Transformers actors should not be judged on their acting ability based on those film. (Or at least, the parts that feature the robots. So, around 50% of the entire films. ;p)