You can sit on the grass at a university or even sit in most places where you can physically sit. But it is not a very common thing to do. I would say the main reasons why most people dont do that is because it would get books and clothes dirty if you sit on most parks and outdoor spaces. I would not want my books and other items to get dirty when I study, and hence would avoid most such locations to study.

Having said that, you can find enough people sitting in the lawns and steps around most indian universities & colleges.