There is no answer to your question. You are only raising this question because you have less percentage in 12th standard. Generally for MBA admission b-schools look for consistency throughout your academic career. That's why they give weightage to all the academic records in selection.

After you give MBA exams like CAT, XAT, CMAT, MAT, SNAP, NMAT etc, then all the colleges in which you apply for admission  makes a merit list to call students for second round of interview and WSAT which is made in the basis of your aggregate score and which is based on different parameters such as CAT marks, 10th, 12th and graduation percentage, work experience, engineer or non-engineer, gender, extracurriculars etc. whose weightage vary according to different colleges.

In this weightage of academic records is generally approx 10-25%. In which 12th class weightage varies from 5-10%. In this first merit list, most of the colleges give highest weightage. So to cover a little loss due to 12th exam percentage you can score very well in CAT exam and you can get calls from most of the colleges.

Also after the second round of WSAT and interview this weightage of academic records is further reduced and a new merit list is made which has the highest weightage given to WSAT, interview and MBA exam marks. So again you can perform excellently in the second round and ensure your admission to the college.

Don't worry about it and prepare hard for the exam. All the best!!