If she was single, the earnings limit for 2020 was $14,050. Since she earned more than the limit a return should be filed — But if there was an estate filed for probate purposes, then the estate would file the return. Did you have a probate attorney handle the settlement of the estate? Were there any other 1099s — other than the one for SS?
Strictly speaking, it is the responsibility of the executor of the estate or successor trustee of a revocable living trust to make the tax filing. But you could wait and see if the IRS contacts you about filing a return. I would also call SS at (800) 772-1213 to see if you can provide a death certificate and have them send you a duplicate 1099.