By studying only the 12th topics you will be leaving 50% of the syllabus thus you will not be able to attempt 50% of the paper due to which you can at max score 180 which is not a very good score. Therefore instead of adapting this strategy you should leave the unimportant topics of 12th and study the important topics of 11th in order to maximize your score. Hope this helps.

You can expect around 55-60% questions from 12th class in JEE Mains. But it keeps varying yearly and subjectwise. Check previous years data for more clarifications on class 11th and 12th part every year. Also, if you are giving JEE Mains next year, do register for CTSE. Link: https://learn.careers360.com/careers360-talent-search-exam?icn=ctse_scholarship_lp&ici=navigation-header_ctse