how accessible things are will depend on how old they are. Anything built in the 1990s would have been accessible at the time that it was built. In general, that means things like having ramps in place of stairs (or in addition to stairs) and if there are viewing windows, there also would be a lower viewing window for people using wheelchairs and
ECVs.
Anything built before 1990 may or may not be accessible. Any renovations done would need to be made accessible if possible. Sometimes things are not possibley to make accessible. Most zoos I have been in have been pretty accessible though. They tend to have ramps or hills and a lot of them have lower windows or view areas because a lot of kids come to zoos.
Do you have a specific zoo in mind?