Button mashing is when you quickly and randomly input motions and button presses. Players may do this if they have no idea how to play. Due to the variety of defensive mechanics, this has always been a poor tactic to use in Super Smash Bros. games.
Button mashing does have some competitive use. For one, button mashing out moves with invincibility frames (such as Bowser's Whirling Fortress or Ryu's Shoryuken) allows characters to escape what little chain grabs exist in Clash. Button mashing also helps to break free from a frozen state, a stunned state, or broken shield state much more quickly.
Some attacks in Super Smash Bros. Clash deal more damage and knockback if the player quickly inputs button presses or control stick twirls, a feature which has been in some other fighting games before. Low mash versions may be useful for combos, while high mash versions are usually useful to kill off a character. Attacks that can be button mashed include: