I agree with malloyem. Elvis seemed at ease in front of the camera, as many rockers-turned-actors do. Obviously the material (i.e., movies) selected for him never gave him much of an opportunity to demonstrate whatever abilities he might have had, but I think if he were given a decent role, he might have shined in it.
At his worst, he probably could have carved out an acting career in the manner Dean Martin did--not as second fiddle to Jerry Lewis (I guess he did okay there, also) but as a reliable cowboy in John Wayne or Frank Sinatra movies. I'm not sure that makes him any more of an actor than "Viva Las Vegas" or "Girl Happy" did, but I think Elvis could have made a living for himself as an actor if he wanted or needed to.