The role of 12th marks depends upon the situation and on the person who sits infront of you while taking aadmission. Some doesnt bother about 12th if you have good percentage in entrance exams(jee). But most of the ppl matters! There may be a chance of rejection if marks are ltoo low. Anyhow try to maintain 80% .Best marks never give you less importance right!

Hey dear they play important role during admissions in top colleges like IITs ,in case you clash with a person with similar score or maybe during calculation of merit based cgpa score for international university or some top private colleges of India consider 12th score as 50% criteria to take out merit based score of an Student respectively.