For context:

Look-alikes are grayscale clones of characters in Super Mario Party Jamboree. Being a variation of the ten Jamboree Buddies, they are exclusive to online matchmaking alongside the Party-Planner Trek mode and appear as stand-in characters created by Kamek. During the Party-Planner Trek mode, several of the playable characters appear with a mission for the player. If the player has chosen one of those characters, their look-alike will appear on the board and in their respective Showdown minigame instead.

Look-alikes also appear in the Minigame Bay modes, during Free Play and Showdown Minigame Battle.

TL;DR: Nintendo didn't wanna program different minigames to replace mario or smth.