Is it wise to leave a group C job of tax assistant at Mumbai Customs for pursuing an MBA where the only calls are from TAPMI, KJ, Great Lakes & XIMB HRM?

It all depends on your goals and aspirations but these colleges will give you a good value for money. If you want to switch and pursue a career in management then it is a good choice to leave the job and enroll in any of these colleges or if you like government jobs and want stability in your career then continue with your current job. Even if you take a loan you don’t have to start paying it back right away, you will get at least a year of moratorium and the student or education loan has one of the least interest rates, worry not. 

For better assistance below is a table showing the fees and packages of the colleges you mentioned;

The data keeps changing and also it will take you two years to complete the course, so in two years the placement data will get better, so RoI is better.