If you'd like something else to dissect for whether or not it's a wavedash, look at the Nintendo World Trailer in Snake's section. Just before he blasts Pit with the Rock Launcher, he bobs his head forward and moves toward Pit slightly. It's not a natural-looking motion, which makes me wonder if it's a wavedash or something else entirely. Sorry I don't have a GIF for it.
As to the percent-based health system, my interpretation of it is that at 100%, a character of average "weight" (like Mario) stands a significant chance of being knocked off by a smash attack of average power. Essentially, 100% damage is where most people will fall off, on average. Obviously, it takes more damage on Bowser and less on Kirby, but 100% is the center point.