Final answer:
Granny believes the person referred to as 'a Fetty' is exceptionally beautiful, a stark contrast to the rest of the Fetty family, whom she suggests to be unattractive.
Step-by-step explanation:
Granny's opinion about the person mentioned as "a Fetty" is that she is exceptionally beautiful. This is evident from the phrase "uncommon purty fer a Fetty," which suggests that the individual stands out in attractiveness compared to other members of the Fetty family, whom Granny perceives as unattractive, implying they "took homely pills." The expression is Granny's colloquial way of contrasting the person's beauty against the perceived lack of it in the rest of the family.