Although I am not aware of a mechanic being charge criminally in regards to a crash it is within the realm of possibility in the United States. Whether you agree or not with the lawyers looking for deep pockets I would think in a case like this they want to affix blame and in some cases, get the maintenance organization or the operator involved as they have the deep pockets (read insurance). I do know of a specific case where Boeing suffered the loss of a commercial CH 47 in Scotland several years ago. The crash was traced to internal corrosion resulting from a design change made in the gearbox. They not only got Boeing involved but they also sued the manager of the design section and one of the designers directly involved in creating the change. No one is immune from prosecution.
On the reverse side of a US A&P certificate it says,” The holder hereof shall not perform or approve alterations, repairs or inspections of aircraft except in accordance with the applicable airworthiness requirements of the federal aviation regulations or such method, techniques and practices found acceptable to the administration”. If something goes wrong and the problem can be traced to the mechanic making an error his ass is grass and the FAA and the trial lawyers are the lawnmower.
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The Cat