Coming from someone who plays goal, no its not a bad goal at all. Its still technically stoppable, but its just below being bar down on the blocker side. Any sane stance will have your blocker hand sitting on top of the paddle, roughly in the middle of the net or a little lower, so you have to move the blocker hand upwards through its entire range of motion to reach top corner. Doesn't sound like much, but its a long ways to go when you have maybe a half or a quarter second to react after picking up the shot. And the shot is incredible. That has to be 100 mph or at least close.
All goalies are weaker blocker side because any save you make with the blocker you have to consider what youre doing with the stick too. Not as simple as making a glove save where you have a free range of motion. Your stick is vital for covering the fivehole and deflecting away shots near the low corners, so if you make a move to stop a shot with your blocker you have to be really sure that youre not about to take your stick blade off the ice.
The only really major mistake Casey made on that play was not being aggressive enough on the angle. Top of the crease isnt enough for a scorer of Yzermans calibre, and if you watch closely Casey fades backwards into his net throughout the shot when it would be better to cut an aggressive angle and hold it.
My addition to this thread isnt actually a bad goal, but its my all time favourite "almost" bad goal:
Wings, Hawks 1995 Conference Finals Game 2, Wings leading by a goal in the final minute
Imagine if Vernon had let a stinker like that in. Itd live on the highlight reels forever