Himanshi, that is not at all a mistake. It is ok if you use your Mother's surname in the application form. That is not a mistake. You can leave the application form the way it is. But to answer your question, FAQs section of DU Admission 2020 portal official website available at https://ug.du.ac.in/index.php/site/faq , question no# 4 says that you can make changes or update information in the application form till the last date of registration. However, there is a correction fees of Rs. 100 that you need to pay towards the changes that you make in the application form. So you can rectify your Mother's name by logging again into your account. Go to the "Edit" Button at the end of each page or session and make the necessary changes. Since July 31st is the last date for filling the application form, please try this out and do  not wait until the last moment. But before that, please reach out to the help desk to check if it is okay or it needs to be corrected at: undergraduate2020@admissions.du.ac.in .