what does "dumb as a bg of hair"mean?
He sampled the lemonade. “Look, Ms. Walsh—Abra. I’m going to tell you how this reads, and how my boss is going to read it. If you had anything to do with Duncan, why the hell didn’t you throw that gun off the cliff, especially after we executed the search on Bluff House? Putting it in your bedroom closet with a bunch of incense? That makes you dumb as a bag of hair, and there’s nothing that indicates you’re dumb as a bag of hair.”Not trusting her voice yet, she nodded.
“You find it, call it in. Coincidentally, the lead detective on Landon’s wife’s homicide gets a call from an anonymous source—on a prepaid cell that pinged from a local tower—claiming, three weeks after the incident, he saw a woman with your hair and body type walking away from the crime scene on the night in question.”
“And Detective Wolfe believes him.”Q 2: what does "not trusting her voice yet"mean?

Question

what does "dumb as a bg of hair"mean?
He sampled the lemonade. “Look, Ms. Walsh—Abra. I’m going to tell you how this reads, and how my boss is going to read it. If you had anything to do with Duncan, why the hell didn’t you throw that gun off the cliff, especially after we executed the search on Bluff House? Putting it in your bedroom closet with a bunch of incense? That makes you dumb as a bag of hair, and there’s nothing that indicates you’re dumb as a bag of hair.”Not trusting her voice yet, she nodded. 
“You find it, call it in. Coincidentally, the lead detective on Landon’s wife’s homicide gets a call from an anonymous source—on a prepaid cell that pinged from a local tower—claiming, three weeks after the incident, he saw a woman with your hair and body type walking away from the crime scene on the night in question.” 
“And Detective Wolfe believes him.”Q 2: what does "not trusting her voice yet"mean?

Answer

In Canada, we often say "She is as dumb as a bag of hammers" and this means that she is not smart at all.
"Not trusting her voice yet" means that she was so scared that she did not trust herself to speak yet because she did not know what words would come out of her mouth.
Hope that helps!

Answer

There are grammar errors in the original text. It should be:

That makes you AS dumb as a bag of hair, and there’s nothing that indicates you’re AS dumb as a bag of hair.

Anyhow, it just means "really dumb", I guess. Usually people here in the States say, "You're as dumb as a box of rocks/bricks." though, which just means "You're really stupid." Perhaps in other English speaking parts of the world, they say "as dumb as a bag of hair" instead.