If nothing else, "Mammie Katie" certainly had an interesting life. The little I know of her raises many questions, but I'll start with what appear to be the cold, hard facts.
Katie Appling Clark is buried in Monticello, in the Appling-Clark cemetery. According to her grave, she was born in February 1829 and died in September 1928.

In the same grave are buried her husband Allin Clark, who died in 1911 at 80 years of age, meaning he would have been born in 1831.
Millie Clark, her mother, is also buried here:
Millie's grave says she was born in 1806, and died in 1918.
This is the 1880 census from Jasper, Denegal, 073

This record shows Allen Clark as a Black male, age 54 or 57, as the head of the household. His wife is Katie Clark, age 42, listed as a Mulatto female. Son Henry (presumably "Clark") is listed as age 21, and a Mulatto male. William Bell, a Mulatto male age 9 is listed as a "boarder". Black female Rhoda (??) Bland is listed as 18, and a single boarder. Milly Cochrane is listed as a Mulatto Female, age 68 as "mother" and as either widowed or divorced.
In the lot adjacent are William Appling (Katie's son) Mulatto male, age 22 with wife Berta, mulatto female, age 21. Their son W.S. Grant is 6 months old and is also Mulatto.
In the next lot is Martha Cochrane--Black, female, 21, single head of household. James, Joe, and John Cochrane, 3 years, 2 years and 10 months of age respectively are both listed as "Black".
On this list, Martha Cochrane stands out to me as a red flag. She is 21, and listed as single rather than divorced or widowed, with 3 sons who are all about a year apart, each. Although she is not a relative to my knowledge, I have heard some things from elder family members that makes me want to keep an eye out for Cochran(e)s, especially those living in close proximity to Allen and Katie.

This image shows a marriage certificate of Catharine Cochrane to Allen Clark on January 13, 1866.
The year and the lack of any notification that either party is of color, make me wonder if they were passing for white, since other marriage records I've seen for people of color have listed that somewhere. Although Allen Clark is listed as "Black" in 1880, in 1870, he is listed as "White", which obviously makes me think he could have been fair-skinned. Still, African-Americans were officially allowed to marry (each other) in April of 1865, so it is possible that Jasper County did not document their marriages in this way.
This is the 1870 Census
It shows Allen Clark as White, 40, living in the household next to Mildred Ann Cochran, widow of Jubil Cochran, former owner of Katie and her mother Millie.
The next page gets more interesting
This page shows Catharine Clark, Mulatto, age 30 (listing her as 10 years younger than the subsequent census) living in the same household as White Allen Clark. Unlike the 1880 census, the 1870 census does not take into account whether people are single, married, widowed, or divorced. It is unclear to me whether or not this means Catharine (Katie) is listed as Allen's wife. I'm inclined to think not, since a white man could not marry a mulatto woman in 1870. It is also worth noting that she is listed as a "domestic servant" which possibly could have been used to explain her presence in Allen's household, if she was not able to be listed as his wife. Alternatively, since it is very obvious that something else was written before the "W" next to Allen Clark, that they were listed as married, and that there is just a paperwork error.
Sons William and Henry are also listed here in 1880, although each of their birth years are listed as 2 years earlier. Sarah Cochran, Black, 18 is also listed in this household, as are Charles (presumably "Cochran"), Mulatto, age 2 (or 12?) and Monroe (??) (again, presumably "Cochran") age 19, also a mulatto male.
I have a picture of Katie but I haven't uploaded into a digital file, yet.
According to oral history, Katie's father was William Johnson, a planter (a.k.a. slaveowner) who was born in 1790. According to what I have found, he died in 1822, which would have made it impossible for him to have fathered Katie. I am still investigating this but so far, these are what would appear to be the cold, hard facts.