I think you need to find a few more records to answer your question.
So Thelma's mother died in 1917, and Thelma is living with relatives in this 1920 census..."United States Census, 1920," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:M4VY-B56 : accessed 17 August 2018), Thelma Coor in household of Mamie A Coor, Encinosa, Lincoln, New Mexico, United States; citing ED 100, sheet 5A, line 41, family 79, NARA microfilm publication T625 (Washington D.C.: National Archives and Records Administration, 1992), roll 1077; FHL microfilm 1,821,077
In 1910, Thelma's mother is in this census, WITHOUT Thelma.... a bit odd. But she's going by her maiden name.
"United States Census, 1910," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:MGS8-H2N : accessed 17 August 2018), Lenah V Mobley in household of Noah A Mobley, Hagerman, Chaves, New Mexico, United States; citing enumeration district (ED) ED 26, sheet 10A, family 226, NARA microfilm publication T624 (Washington D.C.: National Archives and Records Administration, 1982), roll 913; FHL microfilm 1,374,926.
You should probably at least try to find Thelma in 1910 to see what surname is being used. But best of all would be a birth record.