The Torah tells us (Bereshit 25:1) that "Abraham took another wife, and her name was Keturah."
"Abraham married three women: Sarah, the daughter of Shem; Ketura, the daughter of Japheth; Hagar, the daughter of Ham"
In this context, daughter means descendant, and not an actual daughter (e.g. Hagar was an Egyptian, and Mitzrayim was the son of Cham, so she was at best his granddaughter).
Many commentaries on this section in Torah (e.g. Rashi, see here for a list of some others) say that Ketura is Hagar. However, the quoted Yalkut Shimoni can't agree with this opinion, since clearly the Yalkut Shimoni holds that they weren't the same person.
There are opinions who maintain that Keturah and Hagar are two separate people, but of the ones I read (Rashbam, Ibn Ezra, and Ramban), only the Ramban (Bereshit 25:6) says what her nationality was, and he says she was a Canaanite (also a descendant of Cham). So the Ramban isn't saying the same thing the Yalkut Shimoni is either.
According to the Yalkut Shimoni, who was Ketura? Was she someone important (like Hagar, who was an Egyptian princess)?
The same question could be asked about all the commentaries who say Keturah was not Hagar.