Although it is not impossible to plant such evidence it is highly unlikely that we are dealing with planted dna.

First someone wanting to plant evidence is going to do a real good job of it and plant a whole lot, so as to make sure.

Second, degraded dna can easily result from the actions of bacteria from the urogenital tract.

Third, if it is a scratch that drew blood and intruder put his hand to the wound thus getting finger slightly bloody and transferring that to the panties, then fingernails would have epithelial cells and panties would have blood cells. Someone who planted the evidence would have to have obtained each substance from some unknown 'contributor'.