Last night I stumbled onto the Oberlin city website and found a wealth of historical and genealogical material. Among other things, Maria Alexander Gibbs is listed in the city directories from 1873 to 1902—first as "Mrs. M.W. Gibbs," then as "Mrs. M.A. Gibbs." Her daughters Ida and Hattie (Harriet) were sometimes listed as sharing her residence.
I've also found an 1850 Oberlin census that lists several young "mulatto" women from Kentucky named Alexander; I presume they were Maria's sisters or cousins. But I still haven't found any record of her death, or any indication of an occupation. She may have devoted herself to raising the children, and presumably Mifflin Gibbs provided support.
The Gibbses were a remarkable family, and it's frustrating to find so little of their lives is recorded—even Mifflin's life.