Smash can be both.
With items and smash balls, you can still win while button mashing. Good mindless fun.
However, in upper level play where certain fixed settings are encouraged, Smash suddenly becomes anything but a "button-mashing" game in the sense of being able to press a bunch of buttons all the time to beat skilled players.
Not sure what you mean by "full motion control", but the game does demand that you are purposeful and strategic with your button input in competitive levels of play.