Most people do not know what ADS-B is. So if you tell them that you are decoding signals being sent by an aircraft they will get paranoid and say it is illegal. As far as I know, there is no law which explicitly states that you cannot have an ADS-B receiver in India. But I also know that one cannot own a receiver of any kind except for maybe broadcast radio and TV without a license in India. So it is kind of a gray area.
If you have to show this to someone maybe get an SWL (very easy, no examination) or ham radio license (takes a year and need to give an exam) from WPC (Wireless Planning Coordination, Ministry of Communications). Then you can say you were tuning your receiver and you heard these ADS-B signals and you decoded them. Also, helps if you explain the person what ADS-B signals are and how they work.
So, is it illegal? Maybe. But does anyone care? Nope.