it is unlikely that the mechanic messed up anything in your transmission. Auto trannys are never touched when it comes to replacing or servicing torque converters. As a matter of fact, most mechanics will simply take out your transmission and send it to a specialist when it goes bad. Therefore, they are just the middle man who is removing and re-installing it.
So there is a good chance that the transmission just had enough and decided to quit on you. But there is really no way of knowing without physically inspecting it.
congrats of your first swap and good luck with the tranny