Dear aspirant,

By applying for CMI you are getting into a mathematical institute to do a bachelor's in maths. JEE will help you get into engineering colleges of your choice. So, naturally the requirements for the exams are different. The questions in CMI are mostly subjective and of the level of olympiads. It will require more critical thinking whereas JEE mains is mostly about solving the questions on time. The difficulty of the exam can be compared to that of JEE advanced acing CMI would require religious preparation tailored for the exam. As both exams require very different preparation approaches, think about what you want to pursue and then prepare accordingly.

Hope this helps.