Atticus is still very much relevant. Three reasons come to mind.
First, although we have progressed since the days of Atticus Finch or even Harper Lee, racism is one of those intractable evils in our society. Therefore, a book that deals with race and a man who embodies fair judgment ought to be welcomed. More to the point, a man who would go against the grain and stand for what he believes is worth studying.
Second, the context of Atticus Finch in Maycomb might be far from our day and age, but the virtues that Atticus stands for are timeless. Atticus's courage, generosity, and self-sacrifice are amazing and inspiring. Here is one of my favorite dialogues:
“Atticus, are we going to win it?"
“Simply because we were licked a hundred years before we started is no reason for us not to try to win,” Atticus said.
Even within our postmodern framework, virtues like courage and generosity never go out of style. No culture I've ever studied praises cowardice.
Finally, Atticus is also a great father. His commitment to his children is something that can help many as well.