"Doc" Rutledge, the veterinarian who's treating all the animals in the Mutant Wizards' office, is a former member of an (unnamed) militant animal rights organization.
One of the most vivid scenes in the book is when Doc tries to "liberate" office mascot George, the one-winged buzzard. Doc mistakes George for an eagle, and tries to push him out the window. George, being unable to fly, resists this attempt mightily. Finally George does what buzzards do when threatened. He vomits.
We learned that's really what buzzards do when threatened. It achieves two purposes. It makes the buzzard lighter, so they can escape more quickly, and sometimes it distracts the predator by offering them a "free lunch."
Andrews does tend to pillory groups and individuals who take their personal passions too far. With Doc, she spotlights rabid animal rights activists who go too far in their efforts to protect animals, and sometimes actually do more harm than good. Doc becomes a recurring character and turns out to be a pretty nice guy in later books. And Andrews isn't mocking all people who are devoted to animals, just those who carry their passions too far.
Wikipedia has a good list of animal welfare groups that are great groups to check out if you love animals.