Will the laying of mines be considered a crime?

As you might expect, it depends. The legality of minelaying is dependent on the laws and jurisdiction of the area, as well as the authority (or lack thereof) of the organization tasking you with laying mines. You can probably assume that laying mines in the vicinity of a heavily inhabited UEE planet would be a very criminal act. On the other hand, being tasked by a UEE mission giver to deploy mines around a Jump Point to a Vanduul-occupied system would likely be legal. There are all kinds of possible circumstances in between too, which inhabit various legal grey areas.