The good vs. evil split is pretty straightforward in Prince Caspian. In the blue corner, wearing lion gold, consisting of our main characters, are the good guys. In the red corner, wearing Telmarine green, consisting of some devious fellows indeed, are the bad guys. And then they fight.
Yep, there's not much subtlety in the way the novel handles issues of good and evil. The good guys are good, the bad guys are villainous, and the middle ground is narrow. It's pretty simple by modern standards, but that also might be the point. Prince Caspian borrows many themes from classical works—we're looking at you, chivalry—and it doesn't get more classic than a simple tale of good versus evil.
Prince Caspian, and the Narnia series as a whole, uses simplistic notions of good and evil because it's a children's story. As a result, they totally underestimate what children can and can't accept in their stories.
Miraz is certainly our antagonist, but other characters are much eviler. Think Nikabrik. In the same way, Caspian is our protagonist, but characters like Peter represent the "good guy" better.