Ok, first and foremost, tiers do exist, and they are a very significant part of playing a game whether you beleive it or not. Characters can only go so far without being bound to their limitations.
Secondly, Ike is garbage. Sure, we didn't get enough time to use him, but what is there to learn beyong his moves and some of his character specific traits? Beyond that, you get nothing but techniques that every character can use. We aren't saying that we mastered Ike, it's just that he is the only character in the Demo that couldn't make progress as a good character. Even if it was possible to master his moveset, there is no possible way to make him effective enough to land hits at will or in a consecutive manner. Most characters such as Bowser or Ganon have moves that can be used for safe approaches and come out fairly fast. Ike doesn't have moves that allow him to attack safely; A tank that can't hit anything. And don't even mention the super armor frames, because they only exist on his Aether so far.
Think about it, if others were able to make progress with their characters and play them effectively, but others were not, don't you think that there is a problem with the character? I played with Pit, MetaKnight, Mario and Fox and E For All(I made the most progress with Fox despite his nerf), and did fairly well with them through out the convention, but Ike is so horrificly slow that you couldn't imagine how painful it was just to do a simple tilt.
Ike is one of Gimpy's favorite characters, and one of mine as well considering that I love Fire Emblem. Ike is bad, and he can't be compared to Bowser in any way.