what makes someone “good” or “bad”

Film Noel Murray from the AV Club considers whether Natalie Portman can act. The short answer is, of course, that yes she can, but it's useful to see someone weighing up the facts:

"Well, at the risk of sounding wishy-washy, the real quandary here concerns “acting” itself, and what makes someone “good” or “bad” at it. It’s a question complicated every year by awards season, where the showiest performances often win, even though in some ways it’s easier for actors to play a role that requires a lot of scenery-chewing, physical transformations, and outrageous accents than it is for them to make an ordinary human being into someone audiences care about. Plus, actors rarely control what ends up on the screen. They can only deliver the lines the writers give them, while taking the guidance provided by the director and having their performance affected by the chemistry with their co-stars. Even then, the editor can still choose to leave their best takes on the cutting-room floor. Given all that, what are we actually weighing when we consider an actor’s skills?"

Exactly. That's why the argument against Andy Serkis not being considered for awards nominations for his work in Lord of the Rings because he was artificially enhanced in post-production to appear as Gollum was such a misnomer. All actors are artificially enhanced (or otherwise as Murray notes) in post-production.