If the dividends received benefitted your Mom, she needs to report the income. Income with respect to a decedent that is paid directly to a beneficiary is reported on the the beneficiary's 1040. So if the dividends went into the investment account that was later titled in your Mom's name or if the dividends came directly to your mom, they would need to be reported on her 1040.
You could segregate those in TurboTax by placing "IRD" after the stock name so you will know which dividends were reported in your Dad's social security number when 2021 come around.
