Yes, that's it. Some of the characters don't even have names; they're identified and defined by the roles they play in the world of the story -- the Baker, the Prince, etc. But these characters step outside their conventional roles and do things that are not necessarily expected of bakers and princes and the like. Little Red Riding Hood's basket of goodies she's taking to Granny are all conned out of the Baker's Wife or outright stolen, and the girl eats most of them before she ever gets to Granny's house. The Baker's Wife and Cinderella's Prince have a little smoochfest when no one is looking. The Baker himself -- an ordinary, confused and frightened man -- turns out to be a hero in his own modest way.
But the Witch stays within the boundaries of her conventional role; she's evil and powerful from start to finish. We may have gotten a hint of more complex behavior from the Rapunzel scenes. The Witch may have developed motherly feelings for the young girl. That's just a guess; we're not really shown enough to know for certain and there's no follow-up. Streep could perform only the material she was given to perform, and she did quite well with it. But I am curious about the missing bits.