If you lose being spectated in Hearthstone it's a gross misplay, but if you win it's dumb luck.
Truth be told, people aren't diviners to know the next card the opponent will play and, yes, there is a good degree of luck on HS. Surely you can make educated guesses but in the end of the day it's all a roll of the dice and it's easy to look dumb or lucky playing it.