You cannot compare them because both JEE and CAT have different out reaches. JEE Mains is the entrance examination for admission to Engineering courses. It is the entrance exam to study BE/B.Tech/B.Arch in Private Engineering Colleges, State run Government Engineering Colleges including IIITS, CFTIs and NITs. It is also the gateway for admission to IITs for which entrance examination is JEE Advanced. To qualify for JEE Mains, there is no minimum aggregate criteria. But if you want to join NITs, IIITs or CFTIs then you should score a minimum aggregate of 75% only in your 12th. 11th marks aren't considered. And then to appear for JEE Advanced which is the gateway for admission to IITs, you will need to fulfill the 75% aggregate condition. Now it is 75% for General Category and 65% for Reserved Categories. So how much you score in 12th is what matters to secure your eligibility for NITs, IIITs, CFTIs and JEE Advanced.

CAT is the entrance examination for admission to Master's programmes in all the 20 IIMs and as many as 1000 participating Institutions. Through JEE you get to study B.Tech and through CAT you get to study MBA. And you need to have a Bachelor's degree to appear for CAT. So there is no comparision between both the examinations.