There are a couple of points to make here. The first would be that Parris is evidence that justice does not prevail because he is empowered. The fact that someone like him would be in the position of power is representative that justice is not present. Parris' insecurities, his coveting of power and control, as well as the manipulative manner in which he uses religion are all representative of how injustice is present if someone like him holds power. Additionally, Parris' role during the trials also feeds the idea that justice did not prevail. Parris' pressures on Danforth and the entire court proceedings is representative of a lack of justice. His pointed attacks on Proctor, seeking to pervert the court process to remedy his own personal animosity, is also representative of how justice does not prevail.
On another level, though, one could argue that post- trial, justice does prevail with what happens to Parris. It is evident in Act IV that Parris' credibility with the judges is in doubt. When he confesses that Abigail has stolen his money and fled, that he is the source of death threats, and that he is working on a "deal" to use Hale to secure confessions, it brings some level of doubt to his position. This also helps to show him as fraudulent, confirmed by the ending. In the last section of the play entitled, "Echoes Down the Corridor," the ending of Parris does reflect that justice might have very well prevailed in the long term:
Not long after the fever died, Parris was voted from office, walked out on the highroad, and was never heard from again.
For someone that sought to be the center of everyone's attention, such an ending might be just in its own right.