So how come he ended up looking like half an alien from “Mars Attacks”?

It was “too perfect”.  I used to do 3D rendering and animation.  I pay attention.  The problem with a lot of 3D animation (many times referred to as CGI, or Computer Graphics Imaging) when it’s intended to appear realistic is that the element of real-world “weathering” is forgotten, poorly executed, or unable to be mastered.  I can understand this difficulty when an entire movie is dependent on CGI but, as far as can be discerned, the only significant CGI in “The Dark Knight” was the burned side of Harvey’s face.

If you can stomach it, go to Google Images and do a search for “burn victims” (you may have to turn SafeSearch off).  That’s some scary shit, and had they managed something that realistic looking on the Two-Face CGI then maybe his quick dissent into psychotic dementia would have been a little more believable.