Hi ,
Depends on your choice of branches. If you're looking for the Pure Sciences, then no. None of the IISERs mandatorily requires a math background for BS-MS admissions. However, if you are applying for the Engineering Science (EECS and Chem. Engineering) and the Economics programme at IISER Bhopal, then you are required to have a math background in 11th and 12th. Also, as a previous answer pointed out, the Aptitude Test has 25% questions from Mathematics. So if you are a non-math student, you would be well-advised to maybe spend the last couple of months going over a few basics from the 11th and 12th NCERT textbooks, so as to be able to score a few marks at least in the maths section.