No! There's no way Kristen Stewart could ever outshine Charlize Theron!
In terms of the beauty department - Charlize Theron is definitely one of the most beautiful women in the world and comparing Kristen Stewart to her... well I don't see Kristen Stewart as stunningly beautiful, but rather very pretty and girlish I would not give her the title, "the fairest of them all".
And then in terms of their acting - again, Charlize wins. Kristen Stewart has proven over and over again that she is a terrible actress! And why is she so terrible? Because in whatever movie she has been in, she carries the same facial expression throughout the entire movie! As for Charlize... well... the fact that she's an Oscar-winning actress is certainly enough to show just how talented she is; Oscars are very hard to get remember. I don't remember seeing a film she was in where she didn't give a great performance.
So in my opinion, in both the beauty and acting departments, Charlize Theron wins - she is by far a much better actress than Kristen Stewart and definitely beats her in the beauty department. I think the only reason they cast Kristen Stewart in this movie was because of how popular she is with all the Twihards. Just because she's so popular with all the Twihards doesn't mean that she's the fairest one of all or a great actress...
Observation of both actresses.