I have been looking for a good mechanic to do some work on my car. I want to find one who will not make things worse.
Yes.  It is a good idea to look for a mechanic who specializes in your particular vehicle.  There are a variety of "mechanics."  But if you should look for one specific to your vehicle.  
Yeah, I wouldn't want a mechanic specializing in luxury vehicles working on my Ford pickup truck.
Yes; and even within, let's say, a Ford dealership, there will be specialists within that department.  Some may specialize with the mechanical aspects of the vehicle, while others will deal with the electrical aspects.  There is so much specialization these days.
For good reason as today's vehicles are much more complicated than the days there were like 7 components of a car. The computer systems alone requires extensive training to diagnose.
I agree.  Mechanics really should be certified, as they are.  Find a good certified mechanic for your problem or work.