Do people opt for a PGDIM at NITIE Mumbai even after doing an MTech?

If you are asking from a technical point of view, you will not face any issues for doing PGDIM from NITIE once you have completed your M.Tech.

But, in the future, you might be asked during an interview as to why you have pursued a management degree after completing your master’s in a core field. Be prepared to give a convincing answer. 

If you are thinking of joining the industry and then doing a management degree makes sense. It would definitely help you scale up your current job prospects. But, if you are pursuing this degree without any industry experience, it might come off as a little haphazard, like you are jumping from one degree to another. This might confuse your recruiter a little if your final goal is to bag placement in a decent company. 

You can go ahead but think of a good reason as to why you want to pursue this to ensure that you get placed in your future and don’t face any problems.