HELLO,

Yes, you can attempt the IISER aptitude exam (IAT) even with a 53% in your 12th board exams as long as you have applied and the results are not yet declared. However your admission will be subject to fulfilling the minimum eligibility criteria of the specific IISER you are applying to which typically requires a minimum aggregate percentage. You can check the eligibility criteria for the IISER you are interested in.

Hope this Helps!