What does Maniac mean when he says to Mars Bar, "Yeah you're more than bad all right, Mars Bar. You're more than bad. You're good"?

Mars Bar is extremely annoyed, not just for being humiliated at Piper McNab's birthday party, but also at Maniac for taking him there in the first place. When Maniac is originally invited to the party, he agrees to go on one condition: that he can take someone with him. The someone that Maniac has in mind is Mars Bar. He figures that the idea of taking a black kid to a party in the whites-only West Side is so crazy that no one would ever expect him to try it.
But try it he does. Unfortunately, things don't work out all that well. During an impromptu game of "Rebel," a member of the Cobra gang jumps into the air screaming and lands just behind Mars Bar, making him jump. Mars Bar is absolutely furious, and the atmosphere soon becomes tense. Thankfully, Maniac manages to get his party guest away before things turn nasty.
Once safely back on home turf, and finally free from Maniac's protective grasp, Mars Bar lets rip with his anger, hurt, and frustration:

You suckered me. You soften me up with them Pick-peoples, then bring me here. Wha'd you think! I was gonna cry Okay, I come over. I did it. It's done. And don't YOU be comin' 'round no more, ya hear me, fish! 'Cause you ain't only seen me half bad yet.

That's when Maniac says to himself

Yeah, you're bad alright, Mars Bar. You're more than bad. You're good.

Maniac is acknowledging that Mars Bar is a tough kid, and he admires him for it. But even more than that, he's immensely proud of Mars Bar for doing the right thing, the courageous thing, by accompanying Maniac to the party.