Yes, you would just enter it on your mother's return since she was the only one alive in 2020.
She needs to get your father's name and ssn removed from the account as soon as possible so it will be reported in her ssn.
Since your father's ssn was first on the account, she may get a letter of inquiry from IRS about it.
Their 2019 joint return should have reported your father as deceased so IRS won't continue to expect returns from your father.
