Hi Rohit!
You have mentioned in your question that your results in the 11th standard were not very good.  But I think that if you put in all your efforts and prepare well for your 12th standard exams, there are chances for you to cover up. The eligibility criteria requires you to have scored a minimum of 75% in your class XII. So, if you work hard, you will become eligible for JEE. Meanwhile, you also need to prepare for JEE simultaneously so that you do not mess up in this exam. 
But at the end of the day, if you manage to cover up your intermediate marks, there are high chances of you clearing other entrance exams and get a good rank if JEE turns out to be unsuccessful. So, I suggest that you focus on all possible entrance exams other than JEE also so that you have multiple options with you by the time you finish your intermediate. 

Please let me know if this answer was helpful to you and all the best for your future!