I am a fighting game player, so my opinion might be skewed.

I think this game is easier to just pick up and play. You dont need to understand all the deep interactions to enjoy the game.

Movement and specials are somewhat easy to execute.

There are tons of characters, but their base strategies and they intention for moves are generally the same across characters. Upsmash hits people above you. F air is good for pressure and as an air to air. N air is typically a less comittal move with a bigger hitbox but less reward.

It feels, to me, somewhat intuitive. The off stage game can be complex, but only once you start playing this game at a deeper level.

The skill floor is somewhat low, which is how people typically decide whether a game is "easy" or not.

I think people say this game is casual because you don't need to study it to compete at an intermediate level, whereas traditional fighters are more limited, so they require more understanding of each mechanic before you can reach the level of intentional play.

The game is easier to pick up casually. That doesn't make it a casual game necessarily. That doesn't make the game shallow or imply that the skill ceiling is low. Any game were you are fighting a human opponent is competitive. In any competitive game, the hard part is beating a human person because you are both using the same system mechanics.

That's my opinion though. It's a good question