No, it's a very poor argument.
Firstly a time traveller would almost certainly be obeying some sort of prime directive- and trying to pass among us undetected.
Secondly, if they broke the directive and announced their identity (but why should they?) we'd simply disbelieve them.
Thirdly if they showed up with signs and wonders (in a UFO for example) the evidence would be placed in the file marked "paranormal" and ruled out of court.