Civil Servant (IAS/PCS)

In my opinion, there is no direct relationship between your graduation degree and your rank in CSE. Your rank solely depends on your preparation for the examination and how well grilled you are in the various aspects of the syllabus. The essay paper, the 2 optional papers and the personality test are the key to scoring a good rank in this exam and scoring high in these is absolutely essentially if you need to bag a rank below 100.
There is a chance that your optional paper could also happen to be the same as that your graduation subject - for example, Electrical Engineering (B.Tech), History (B.A) etc. If you have been able to grasp all the nuances of the subject during your graduation studies, it would definitely help you with your preparation for CSE for the same optional subject. But merely your graduation studies would not suffice to score good marks in the optional paper. You would need to do a lot of answer writing practice, analyze the previous year question papers and also study some topics in the CSE syllabus which may not be covered during your graduate degree course.
Some people choose graduate degree courses which are not very time demanding, as they can they can utilize the free time for CSE exam preparation. So if a career in civil services is your ambition, you can choose such a course which would let you spare at least 4 hours a day for CSE preparation. So by the time you would have completed your degree, you have also completed your CSE preparation and can give the exam straight away.Choose the field which you like the most, because CSE first requires you to be a professional in your field before you sit for services. Professionalism is what distinguishes civil servants from the remaining staff. You better long far that. Remember that the commission never expects you to be a academician or expert in GS, they just check for basic know and most importantly run tests to see if you possess the qualities a officer needs, as simple as that.