what does "dumb shits"mean?
The break-in was one thing, and bad enough. But the assault on the woman? That tipped the scales.
But he’d give the client a chance to explain. Sometimes the dumb shits just watched too much TV, got in over their heads, and God knew he’d had dumb shits for clients before this.
So they’d clear the air, and he’d make his position just as clear. No more bullshit. Leave the investigating to the professionals.2. I don't get this part: Sometimes the dumb shits just watched too much TV, got in over their heads, and God knew he’d had dumb shits for clients before this.3.what does "got in over their heads"mean?

Question

what does "dumb shits"mean?
The break-in was one thing, and bad enough. But the assault on the woman? That tipped the scales. 
But he’d give the client a chance to explain. Sometimes the dumb shits just watched too much TV, got in over their heads, and God knew he’d had dumb shits for clients before this. 
So they’d clear the air, and he’d make his position just as clear. No more bullshit. Leave the investigating to the professionals.2. I don't get this part: Sometimes the dumb shits just watched too much TV, got in over their heads, and God knew he’d had dumb shits for clients before this.3.what does "got in over their heads"mean?

Answer

'Dumb shits' is basically an insult, it's more extreme than calling someone 'dumb'. 'In over their heads' is an idiomatic expression that means that someone got into a situation that they couldn't handle.

Answer

"Dumb shit" is an insult meaning that the people referenced are stupid and not very nice. Getting in over their heads is to take on a project that they are not capable of completing -- it's too difficult for them. You could also say that they have bitten off more than they can chew.