I noticed on my recently fired brass that all of my case mouths are deformed (bent in)on one side. Is this from them bouncing off of the brass deflector, or is it something more serious?

Is from doing the bounce off the deflector.

If your planing on reloading the cases, you can get a piece of velcro(bushy side) and apply it to the deflector to cusion the brass when it hits the deflector.  

I have a brass deflector and my shell casings come out in perfect condition. My friend has a match AR with no brass deflector or forward assist, and the mouths on his cartridges come out dented on one side.

I have a brass deflector and my shell casings come out in perfect condition. My friend has a match AR with no brass deflector or forward assist, and the mouths on his cartridges come out dented on one side.

There doesn't have to be a brass deflector present to ding the mouth of the case.  I have a Min-14 the dents the mouth every time.  My Bushy aslso does this.  My Colt MT6601C never damages the brass.  Both AR's have the deflector.  

I think it has to do with the force of ejection.  If the ejector spring is strong enough I think if causes the mouth of the case to rotate far enough backwards to hit the deflector before the case has a chance to clear the deflector.  Then the case obviously bounces off the deflector.