The Brno Zoo is located about 25 minutes away from the city by tram. It is very easy to get to as well. From the main tram stop across the street from the bus station, hop on tram #1 and take the tram that goes away from the bus station.
There is a zoo stop on the tram. Cross the little bridge across the river, and follow the signs. The walk only takes 5-10 minutes.
The zoo is on the smaller side, and is somewhat hilly. I am not a big fan of zoos because I think they harbor animals unnecessarily in most instances.
The good thing about the Brno Zoo is that almost all of the animals have a large area to roam around. You can also get very close to some of the animals, like the zebras and camels.
If you have an a few hours to spare, the zoo would make a decent side trip, but nothing that I would plan my trip around.