If someone is getting final admission into JBIMS, should he get it or wait for next year to outperform the CAT and get into IIM?

Rather than answering your question subjectively, I have listed some major pros and cons of the institute that makes it a great choice for an MBA.

Pros

1. The small batch size and good quality of the crowd ensure excellent placements, especially in Finance. With only 120 students, the chances of almost all the students securing a good package are high.
2. JBIMS does not negate average profiles and many students with much caliber and talent but average academic records get a chance to showcase their knowledge.
3. Owing to its splendid location, many industry experts visit the JBIMS campus for sessions. These visiting faculty add a practical element to the learning experience of the students.
4. Again its location comes into play as JBIMS provided great interaction with alumni and other industry experts 
5. The alumni are very cooperative and are ready to offer assistance at any stage. There is exceptional harmony among the alumni
6. JBIMS has very low fees compared to IIMs

Cons

• Lack of hostel life which is very crucial 
• There are no major placements in Private Equity or Investment Banking - Front End roles. So if you want PE jobs then only IIM A, B, C can do justice.

I will say that forgo an offer from JBIMS only if you are confident that you might clear CAT and make it into IIM next year.