They did a lovely job.I liked this movie a bunch. However...
The reason the lack of birds and reptiles bothered me is because this film is a deconstruction of other funny animal films like Robin Hood, Cats Don't Dance, and Kung Fu Panda. Those films had plenty of bird and reptile characters, and excluding them makes this film feel unfaithful. Imagine a Star Wars parody that excluded Darth Vader, for instance.
In fact, even though I liked the predator/prey relationship, I feel the anti-racism message could've been just as strong, if not stronger, if birds and reptiles were included.
Here's a neat drawing by squeedgemonster that interprets what reptiles would look like in Zootopia.
Also, for you guys surprised about Mr. Otterton going savage, otters can be surprisingly vicious in real life. Go look it up online.
(Though a part of me can't help but feel that the gator is the littlest bit too exaggerated to fit in with the rest of the Zootopia animals.)