Skull Kid needs the mask. I feel like this is self explanatory. It doesn't have to centralize his moveset. But it certainly gives it flavor. I want to show what I mean here. May clean this up later.
Neutral B:
"Forest Curse"
The power of Majora's Mask is capable of cursing opponents. This can be conveyed in Smash as a dark magic projectile that, rather than inflicting damage, causes vines/roots to seize the enemy and plant them to the ground. This is very similar to the burrowed status effect. If the projectile hits an airborn opponent, they will be swept upward by a swirling whirlwind of leaves.
Side B:
"Eyeball Spinner"
Majora's Wrath will throw razor tops at Link that have singular eyes decorated on top of them. In Skull Kid's moveset, by holding side and B he can hop aboard one of these and ride it like the Spinner from TP. By releasing B, Skull Kid can disembark the top and let it behave on its own (like a red shell) until it eventually slows down and disappears.
Down B:
"Puppet Summon"
Skull Kid summons puppet enemies in the Sacred Grove of TP. Because these are fragile opponents, range and aggressiveness should compensate. They can be knocked out in a single hit but require little to no charge time in comparison to, say, Zelda's phantom.
Up B:
"Shrieking Insanity"
This is not a recovery. Skull Kid will hover in the air and charge up a ball of darkness, all while in his iconic hovering stance and laughing maniacally. Imagine his mask shaking and the purple/orange electric effects accompanying this. Releasing B will throw the projectile at a diagonal/downward trajectory. Impact will scatter the projectile into a dozen smaller ones that fly out in unorthodox patterns and deal minor damage. This B move may not be a recovery, but it does allow him to hover in the air with only a gradual effect of gravity.
Grab:
"Tentacle Tether"
Skull Kid's grab is the longest tether grab in the game, however slow. This is due to his lack of recovery, meaning that he will be tethering back to the stage for the majority of the game.
Forward Smash:
"Beam Attack"
Majora's Mask will charge up a bright orange laser beam and fire it at the ground (think Snake's forward smash).
Down Smash:
"Lost Woods' Protection"
A small cyclone of leaves will whirlwind at Skull Kid's feet, dealing rapid damage. This has a suction effect.
Up Smash:
"Roundhouse Imp Kick"
Skull Kid unleashes an intense break-dancing kick that also partially utilizes his self-levitation to rise up and launch opponents skyward
Neutral Air:
"Spinning Whips"
Skull Kid wields dual tentacle whips that protrude from both wrists. He spins his whole body while doing so, whipping nearby targets with rapid damage.
Forward Air:
"Dark Palm Burst"
Skull Kid sticks both his palms out and causes a contentrated burst of dark magic to appear in his hands, propelling him backwards. Small hitbox but powerful damage.
Back Air:
"Flute Blade"
Skull Kid draws a needle from his flute and stabs it into the air behind him. This is a killing move at high percentage.
Up Air:
"Leaf Gust"
Skull Kid swipes his arms above his body and causes a flurry of leaves to dice opponents that are directly above him. This is not a kill move, but will inflict damage and enough knockback to juggle.
Down Air:
"Dark Slam"
Skull Kid blasts opponents beneath him with a powerful dark magic concussive attack that also slightly propels him upward. This is a laggy move with oppurtunity to punish, but is one of the more devastating meteor spikes.