Now, following the classification for The Legend of Zelda: Echoes of Wisdom it seems the ESRB has now also rated the new Mario Party game. It's rated 'E' for everyone and contains "mild fantasy violence" and notes "users interact" and "in-game purchases".
"This is a collection of mini-games (party games) in which players compete to earn stars, coins, and rankings. As players engage in different modes (e.g., board game, rhythm games, puzzle-based games, flying games, mini-games), they encounter or participate in whimsical instances of fantasy violence: cartoony cannons/cannon turrets that shoot at giant characters; avoiding a robot with giant boxing gloves/fists; using over-sized hammers against each other; connecting electric circuits to zap a boss character. Damage is indicated by blinking, comical yelps, and/or dizzy star effects."