Hi Bhavya, 

   Chemistry makes up 25% of the syllabus of NEET UG, so it is important. All three portions organic, inorganic, and physical chemistry are crucial to get a decent score in NEET-UG. 

  As per your query if it's necessary you study from NCERT, then yes I would not leave NCRT unread because it contains the basic concepts that you need to learn before you tackle the exceptions and tougher questions of medical level entrance. For AIIMS I would recommend you to buy practice papers book as the pattern differs from NEET UG. AIIMS UG exam contains a general knowledge section as well, so please read about current affairs four months before the next AIIMS exam.

 All topics and chapters are important, please don't leave out any chapters because these exams are a mix of all these topics. They are unpredictable.  Study well. 

Hope this helps!

All the best !