Damaging AT-ATs on Hoth
Yes, there is a weak spot, on the underbelly of the AT-AT. But just the very narrow cylinder going along the center underside of it. You pretty much have to be standing right underneath it to hit it.
The safer option is probably to stock up on Ion Grenades, Ion Torpedoes, and Ion Shots. Or go for the difficult but quick T-47 take down.
(Note: This was a change from the beta, when it did not have a weak spot.)