Is it ok if I write 2.00 instead of 2 in the JEE MAIN 2020, since there was instruction only to round off numbers which have more than 2 decimals upto 2 decimal places. but nothing was said about integral values

It depends on the question really. If the question was from the topic of accuracy and precision and the answer had two zeros after the decimal then you are right. But otherwise I doubt the correctness of your question. You are allowed to enter zeros after decimal only when you are very confident of your accuracy, otherwise it is a blunder.