The society of Maycomb is far from perfect, though the town is populated by people who are generally good. This is as true of the juries that Atticus describes to Jem as it is of the society at large.
Prejudice and bias relating to race and class are powerful forces in the society of Maycomb, as evidenced by the events surrounding the trial as well as the trial's verdict. The trial, in all its aspects, is a reflection of the culture of the town, as Atticus implies in talking with his children.
He tells Jem that the thing that happened had happened before and would happen again.
Though one member of the jury makes a strong argument for Tom Robinson's acquittal at the trial (and this is seen as a "baby step" toward progress), the jury returns a guilty verdict in a case where the man's only "crime" was expressing pity for a "white woman".
People like Atticus Finch and Miss Maudie comprise one end of the social spectrum in Maycomb, but they are not in the majority.
At one end of the spectrum are people who fear and hate, such as the members of the jury who convict an innocent man of rape because of his race.
Outside of the jury, characters like Mrs. Merriweather and Mrs. Dubose demonstrate the idea that prejudice is rampant and entrenched in Maycomb. The jury, then, is representative of the population and its views.