Maybe being a glass canon has certain other meaning in other games, and maybe it does in this game as well, but the problem I have with it is that many players can't seem to realize that this generalized term does not determine how you play as Mewtwo. I have had multiple arguments with people who claim Mewtwo is terrible, because he's a "Glass Cannon" yet he is not fast enough, has weird attacks, or is not strong enough to take the offensive.They are of course right about all that, but are also wrong about assuming Mewtwo should be played like that in the first place.
Also, to what Safcar said, Little Mac is more of a true glass canon, because he has the speed. In Smash, speed does determine a character's offensive capabilities, not just strength. All of Mewtwo's power is best used when you have an opening, because you defended yourself well enough to make your opponent fail. Little Mac can sneak in attacks much better than Mewtwo because he's fast enough to do so. Also, comparing one game to a completely different game doesn't really help.
This also brings something else to mind that I just want to mention. I do not care at all about a character's traits in the games that they originate from. So when people say Mewtwo should be much heavier than he is, I disagree (I do believe he should be slightly heavier though). If characters were accurate to the games they come from, then Ganondorf would be Triple-S rank on every tier list ever.
And to what ToTs said, Glass Canon equaling Powerful and Fragile is not accurate. It more accurately means high attack and low defense. Mewtwo may not have weight(fragile), but his defense is amazing.
Trying to play Mewtwo like your playing one of the top characters will get you killed. You have to pace yourself, and counter your opponent. So many people think Mewtwo is just a messed up and poorly designed character because they are convinced they should be playing him the wrong way, and they do that because they don't understand what people are calling him when they say he's a glass cannon.
Maybe he can be called a glass cannon when considering certain definitions of the phrase, but I unfortunately don't have that ability. Where I'm coming from on all of this, everyone I've had to talked to about Mewtwo(which is a lot of newbies on twitch, youtube, and miiverse) believe Glass Cannon means an offensive style, so I'm stuck dealing with it as that. I just believe we should be calling him something else, or at least explain his actual abilities better, to help the players who don't have a clue.