I agree with Ned's logic: Poor housing for humans is called a slum. Therefore poor housing for captive animals could also be called a slum. As I've said before, it is a grey area and there is definitely no clear right or wrong answer here, but even at some of the worst zoos the keepers are attempting to make a difference or are striving to keep an area clean for a particular animal. Zoos with junkyard exhibits or crappy, outdated cages might have keepers who are just as diligent at their job as the biggest and best zoos in the world, but the enclosures that the animals spend their lives in can go a long way to fitting a "slum" description.
Baldur also has some excellent points, including the fact that advertisements can often be quite deceptive. An specific image of a zoological park can be presented that does not contain an iota of truth, and at the same time a bad impression can be attained when it is not warranted. I do believe that complaing and criticizing certain substandard zoo exhibits in the western world has led to the renovation or total destruction of such enclosures. Many American zoos that I visit, and this must surely be the same throughout Europe, are demolishing their outdated cages and grottoes and replacing them with modern habitats with natural substrate. I've visited a couple of zoos in the past where there was a customer survey that asked a number of questions about the overall zoo experience, including what improvements could be made. Years later those two zoos have interestingly enough smashed their old cages down and replaced them with contemporary habitats.
Even the big zoos are not immune to critics, and another intriguing fact is that just before the 1996 Olympics Zoo Atlanta demolished their row of bear grottoes. The bears were all placed in other zoos, the pits were filled in and destroyed, and I'm not sure if anything was actually ever built in their place. I suppose the administration of Zoo Atlanta thought that it was better to have zero bears at all then to showcase such magnificent animals in substandard and outdated grottoes. I know that not everyone will agree with what the zoo did, but I applaud them and really support such a movement. It gives the wrong impression to the public if animals are kept in outdated grottoes and pits, as there are so many wildlife documentaries on every day of the year that illuminate the fact that wild animals come from diverse, beautiful, natural environments. Zoo Atlanta found that people were complaining about the plight of their bears, and so the entire set of grottoes was given the green light to be destroyed. Who really wants to see such "slums" in zoos?