Mona doesn't get enough love. She could easily have an interesting mechanic in *cough* REAL TIME COSTUME CHANGE. If you've ever seen her intermission in Wario Ware: MM, or played Pocket Fighter, you know what I mean. IE, when she does a move that requires her to be in another costume, she instantly appears in that costume then instantly snaps back to her normal outfit. As I understand, she has at least the following ones: Normal, Gelateria, Diner, Band member, pizza delivery, and cheerleader. With those in mind I came up with a moveset for her.
Standard combo: A A A (normal outfit)
Punch, knee, then swipes her robe/cape forward
Tilt A: (Diner costume (Mega partygames)
Swipes a tray forward
Down Tilt: (Gelato costume) Pokes forward with a little dish of gelato
Up Tilt: (normal costume) hops up a little, trying to bonk with her helmet
Smash A: Love Axe (Band costume)
Brings her guitar down business end first vertically
Up Smash : Baton twirl (Cheerleader costume)
Throws her baton straight up where it twirls a bit as she takes a pose
Down Smash: (Pizza Costume) has two boxes of pizza in her hands and kneels, facing the screen and smirking as she slashes in both directions with them as if they were swords
Dash A: (Diner costume) suddenly appears with a handful of plates and trips, causing more damage if the plates break on the target
Nair: (Normal costume) Mona spins in the air, doing damage if people touch her cape (sort of like Marios cape spin in SMW)
Uair: (Band costume) Rears back with her axe, strumming a chord and letting out a soundwave upward
Bair: (Pizza costume) Swipes her pizza box behind her, narrow range but good damage
Fair: (Cheerleader costume) Rears back with her baton and brings it down with both hands, spike if hit with it in lowest position
Dair: (Gelato costume) Flails a bit in the air and drops down a bit faster, narrow hitbox consisting of her lower hip, but always spikes
B neutral: Hot slices (Pizza costume)
Semi-clone of shiek, reaches into her pizza box to retrieve up to three slices that she can throw all at once,doing cutting and fire effects if they hit. Unlike shiek, if she charges up in the air she can throw them horizontally. Unable to save charge
Down B: Soundwave (Band Costume)
Strikes a pose that you can hold and sidle to the left or right. When you release, a circular sound wave envelops her. It can do fair to medium damage and knockback, and reflects projectiles wildly depending on what angle they hit the soundwave.
Over B: Team Spirit (Cheerleading Costume)
Throws her baton upwards, then slides on her knees (on the ground) or airjumps (in air) to catch it where it lands. Can hit with her body or the baton, but if the baton is knocked away will cause her to stumble.
Up B: Scooter Bail (Normal costume)
She rides upward through the air at a sharp angle, then jumps back to bail as her scooter inexplicably explodes. You could use Over B after this to gain more height. Can hit with the rising motion or explosion of the scooter, but is kind of unweidly
Final Smash: I'm late! (Normal costume)
Gets on her scooter and rides everywhere, with her monkey, elephant, and pig on the back of it. The monkey drops banana peels, the elephant throws soccer balls, and I forget what the pig does (scans for weakness?)
Grab Attack: Punches the enemy in the gut
Throw Down: Wraps the enemy in her coat, then spins them around, leaving them standing and momentarily hitstunned.
Throw Back: Drops the enemy behind her and falls onto them with a reverse elbow drop. Leaves them prone and able to roll
Throw Forward: Leaps backward and kicks with both feet, landing on her back and knocking the enemy pretty far foward.
Throw Up: Tosses the enemy up then gives them an action-packed uppercut!
As far as her stats go, she should be pretty average in weight, speed, and power. Her advantage would be her quirky, Game-and-Watch style of fighting where unpreditcability rules.