Be clear that not everyone is genius. Moreover, for Civil Services Examination, you need not to be master of all; if you plan your studies and formulate a strategy keeping in mind your strengths and weaknesses, you too can crack this examination.
The best part is you have clearly assessed your situation and accordingly can prioritize your preparation-plan.
Being average in studies is not a serious threat to your foray in to Civil Services Examination; at the most it can affect is that you may have to put little more extra effort.
Preparing for Civil Services Examination with your post-graduation should not be a problem rather these subjects would be helpful in your preparation.
Keep on moving slowly but, steadily. Determination and Perseverance will help you sail through.
All the best!