Regardless of the year in which you physically received the 1099R, you should declare the income on the return for the year for which it is specified as income (which should be specified on the form itself as a 1099R for tax year 2020 or 2021). If her 2020 return was already filed and this is a Tax Year (TY) 2020 1099R listing income that was not declared on that return, technically you are supposed to file an amended return for 2020.
There are other issues if this is a 2021 disability payment erroneously made for a time period after her death (example for Jan 2021 when she already died in 2020). If she (or her estate) received a Jan 2021 social security disability payment and she died the month before, her estate is actually supposed to refund that payment back to the government. Normally as soon as her date of death is reported to the Social Security Administration (by you, the funeral home, medical examiner, or normally from a combination of these), the SSA is normally quite aggressive about quickly identifying funds they dispersed after the person's death and contacting the financial institution where the funds were deposited to claw back all or a prorated portion of the deposit. I experienced this personally with a social security deposit made into my late father's account about five days after his death. Even though I had reported his death to the SSA within two days, the SSA had already initiated the payment for the next month. Within a week they clawed it back. This might have failed if you had already spent the funds for funeral expenses or other estate expenses and the balance was possibly insufficient or if there was a delay in the SSA being notified of your mom's death.
