What I love about this activity is that my kids need to look carefully at an image, and then do their best to replicate it. They are practicing simple motor skills in their pasting and placement of each piece, and they are also exercising their recognition of shape, negative space, proportion and anatomy.
I like having them write out the names of the animals that they create to label each creation (extra practice in writing, spelling and animal identification).
I like having them select or create their own skins for each animal, as well as the background color for each, so that they have to consider patterns and colors.
I have never stressed the importance of getting each animal "right". I've always tried to keep each assembly fun and easy, and I'm still impressed with how much improvement I see every time we do a new animal.