Yes, the defense absolutely can drag someone's name through the dirt even if they didn't do anything. Sometimes a defense attacks the victim for their action or inaction, or family of the victim -- which we aren't seeing here so far. The defense does this with their client's approval.
I am starting to really wonder about the human feces now. Worley knew there was or would be an intense search for Sierah going on. Certainly he is no criminal genius. He might have expected if/when she was found that the feces was the first thing the crime scene investigators would see and test. I do think it is plausible he planted the feces and possibly someone else's DNA under her fingernails. He may have been trying to set someone else up.
But Worley is no criminal mastermind. The evidence implicating him is absolutely everywhere all over 3 different crime scenes. That they found degraded and random unknown DNA profiles is not uncommon. The prosecution sounds like they are doing a decent job on cross-examination pointing out the problems with DNA collection in a real-world investigation.