It is better to take up a job than do another course before you join an MBA programme. Most good B-Schools give weightage to candidates with work experience than freshers provided you also score the minimum cut off marks. 

Hi

If you sincerely want to prepare for the CAT, in my opinion, then you will not be able to pursue any extremely demanding courses as such.

However, a few things you can do to justify your gap year and add weight to your profile are:

• Pursuing your hobbies - You can do something on a large scale that will impress the interviewers. For example, if you are a good dancer, you could do a certification in that.
• Part-time work - Look for jobs where you spend 3 or 4 hrs such that you can use rest of the time for studying.
• Service or Charity work - You can join an NGO to teach or do some social work

If you are considering a gap year, do read this page once

https://www.careers360.com/articles/gap-year-always-have-back-plan

And don't worry, it is not a bad idea at all to take a year off for preparation.

Regards