Is two episodes too much? (I thought one was.)
I'm more inclined to believe that Lucas made Zam a shapeshifter simply because he could, much like anything else in the worst of his movies. Aliens serve no real purpose but to remind the audience that this is "a galaxy far, far away," as if we'd forget it after reading it in the beginning of EVERY FRICKIN' MOVIE.
Otherwise aliens are totally arbitrary to the Star Wars Universe. They're second-class citizens; slaves, slave-owners, gangsters, and villains. They're social stereotypes that do nothing to serve the story. Even Yoda's the stereotypical homeless guy, dismissed by Luke because of his appearance. But Yoda could have just as easily been human. There's no reason for him to be an alien.
Unless Lucas explains WHY Zam's a shapeshifter in Episode III, she's just another on a long list of useless additions to an otherwise interesting story.