The Boo you mentioned, or rather, the item that transformed Mario into a Boo in some games, would actually be a good idea - but more as an item than an assist trophy.
Captain N: The game Master: Using the pause button on his belt, freezing everybody on the screen for a couple seconds, except the one who called him.
Shantae: Either using her hair as a whip like in Pirate's curse, or one of her transformations.
Alex Kidd: Forces everyone (including the one who activated the Trophy) into a game of Jenken (Rock-Paper-Scissors). Whoever looses to Alex Kidd gets a Megaton punch from him, potentially costing a life.
Kammy Koopa: Blasting your ennemies with some spells for some time.
Baby Mario: Is in his bubble and crying annoyingly loudly, just like when you lost him in SMW2: Yoshi's Island. Everybody looses health or racks up some damage as long as he's crying.
That last one is brilliant! XD
Here are a few more from me.
Urbosa from Breath of The Wild: Unleashes lightning that hits everyone except the summoner.
Rambi the Rhino from Donkey Kong Country: Charges through any opponents who don't jump out of the way.
Mad Car from Earthbound Beginnings: Spins wildly to release smog that gives opponents a Lip's Stick-esque effect.
Roll from the Mega Man franchise: Gives the summoner an E-Tank to heal them.
Nia from Xenoblade Chronicles 2: Tackles opponents and scratches them while shouting "Miii-YEH-YEH-YEH-YEH-YEH-YEH-YEH!".