is CET exam important to do BSC in psychology after 12th
See it is not compulsory if you seek admission in BSc Psychology. Most of the universities offers you admission on the basis of 10+2 merit. But some seeks entrance tests organized by itself or any other entrances in which case CET can be required.
So its specific on the university you want to take admission in.
For example, Christ University, Bangalore offers admission on the basis of CUET. So the CET yes or no is dependent on the university you choose.
To know about universities offering Psychology courses visit the link below:
https://university.careers360.com/colleges/list-of-psychology-universities-in-india
I hope this answer helps. All the very best for your future endeavors!