Graduation marks are not a hindrance for getting selected into the service. However, it would not be correct to say that it doesn’t matter at all.
The interview board is there to judge your overall personality. In assessing that, they have before them the DAF and the summary sheet that you filled during the mains application stage. When they see a poor record in academics, it might make them curious. Why have this candidate scored so low in his studies? Is it because he was not serious about his subjects? Or was it that he was so active in other areas that he devoted importance to those activities too? A curious member might pose these questions to you. If you able to satisfactorily respond to these questions, then graduation percentage wouldn’t matter.
The best trick that can be used there is to be honest. You can be very open and say that in college you found more freedom to explore other activities and hence academics took a backseat. College is not all about the marks that you score. It is also a place where you discover your interests and yourself. If you have the gift of the gab then by all means you can go ahead and tell them these things.
If you look at the technical side of it, UPSC doesn’t give a hoot how much you scored in your graduation as long as you have graduated with the minimum required marks.