I would say this depends on a kind of circus.
There is a number of circuses (the famous cirque du soleil) that do not employ animals and generally do not cause suffering to anyone: there is no reason not to visit those.
However, one should not visit the circuses that use and abuse animals: if one visits those, then one contributes, encourages and supports animal abuse. If the directors of these circuses did not expect people to visit, they would not employ and abuse the animals
