Lorewise, It IS a bit complicated by the fact that the Adeptus Mechanicus is technically a nation within a nation, and handles matters with misbehaving tech-priests internally. All of the characters we play as were convicted through the Imperial justice system, but unless some major paperwork problems were had or the Mechanicus just decided “not my problem”, it’s unlikely that they’d be aboard the same ship that a bunch of normie criminals are on.
Obviously techpriests misbehave, I mean what else are Hereteks, but it’s less about “would a mechanicus character even be a criminal in the first place” like your post suggests, and more about “would the mechanicus allow their units to be tried in the Imperium”.
My justification for this would be that the Mechanicus has less of a presence in the Moebian Domain than other regions of Imperial space, and as a result junior tech-acolytes that commit a crime are handled by the Imperium instead of the Mechanicus to save on resources. The rejects you would play as would be said tech-acolytes, and thus “human enough” to go with the rest of the prisoners, and end up on the Mourningstar’s suicide squads.