Hey Vishnu !
Yes, the Medical Certificate is an essential document at the JOSAA Counselling procedure every year.
You can download it easily and can get it filled up by any registered doctor who puts his stamp upon it along writing down his registration number. This is quite a must and without the Medical Certificate, your procedure of counselling may get tedious and problematic. Along with that, if certain tests are needed to be performed like X-Ray, Eye checkup, blood test. It's advised to do them at the earliest.

Hope this helps.

Good Luck !