It wasn't me that her comment was aimed at (or the wolves). Instead, it was to a middle-school aged boy standing next to her, who had, for the past several minutes, been howling his head off.
I could understand why it annoyed her. I didn't even notice it at the time, having effectively tuned it out. When I see wolves, I generally assume that someone around me is going to start howling.
People visit the zoo for a variety of reasons, one of them being to connect with animals. That may mean watching a training session, feeding a lorikeet or giraffe, or running your hands through an aquarium touch-tank. For some visitors, it also means communicating with the animals. Having a conversation (even a one-sided one, as they usually are) with a zoo animal makes a powerful impact on many visitors.
The sounds of a zoo are one of its top sensory experiences, ranking just behind the sights... and a little above the smells. When a big cat starts roaring, or siamangs start whooping, you can see whole crowds of people rushing to the scene, desperate to see it before it stops. I was at the National Zoo once when the lions started roaring, and a woman literally sprinted from across the zoo to reach the railing. She later told me that she'd been trying to catch the lions in the act of roaring for years, but they'd always stopped just before she got there... until today, her new-favorite zoo visit.
It's not surprising that zoo visitors often try to jump-start these conversations with zoo animals by starting the vocalizations of their own. Usually, they fail. People aren't very good, by and large, at mimicking animal sounds. Partially it's not knowing what sounds to make (who decided that monkeys actually say "Ooh ooh aah aah"?). Part of it is not having the voice for it. I've never seen a wolf in a zoo respond to a howling human.
There are a few exceptions. Kookaburras come to mind. The call of a kookaburra is a riotous peel of laughter, seemingly from the high security wing of an insane asylum. I once worked with an education ambassador kookaburra who could be coaxed into calling for visitors. The visitors would then, in turn, laugh in delight... which would set the kookaburra off into deeper bouts of laughter, and so on for ever.
Parrots are fellow avian chatterboxes. I've seen guests spend several minutes at a time trying to coax a bird into saying "Polly Want a Cracker?" a phrase which, I'm just now realizing, I've never actually heard a parrot say in real life. More often, what happens is the parrot screams, and small children (usually boys in the 6-8 year old range) scream "Be Quiet!" back at the bird. Now, when you yell at a parrot, it doesn't think you're mad at him. He's thinking, "Oh, cool, now we're both yelling! Let's see who can yell the loudest!" The little boys never win.
Some zoos take a more professional view towards animal vocalizations and use them as a form of auditory enrichment. At the St. Louis Zoo, for example, I was once amazed by what seemed to be ventriloquist spotted hyenas. I always heard whooping and cackling, but never actually saw their mouths move. That's when I learned that what I heard wasn't them... it was for them. A recording of (wild) spotted hyenas was played for the zoo's pack periodically, which triggered alert, territorial behavior in the zoo hyenas, who then set about to search for the interlopers and make sure their exhibit was secure.
So do visitors making animal noises annoy the animals? I'm pretty sure the animals don't especially care, unless it's just really getting out of control (too long, too loud). To the zoo animals, unless the visitor is really talented, it's probably just another meaningless sound coming from people, no different than our words. It tends to annoy the keepers a lot, but that's either because they resent the efforts of visitors to talk to the animals, or they're driven crazy by how wrong the sounds are.
When you visit the zoo, remember to enjoy the sounds of the animals, just as much as the sights... maybe more than the smells. Don't feel it's too necessary to add your own, though.