The NP ratio tells you how many Neopoints you'll earn for every point you score while playing a game.
For example, if the NP ratio is 100 NP for every 150 game points scored, to earn the maximum 1,000 NP reward for playing, you'd have to score 1,500 points during the game.
On the other hand, if a game's NP ratio is 25 NP for every point scored, you'd only have to earn 40 points in the game to get the full 1,000 NP for playing.
Mop 'n' Bop:
Petpets keep wandering into the Hall of Heroes, leaving trails of mud from their dirty feet behind them. This angers the Yurble janitor, who uses his mop to whack the offenders, sending the dirt and mud flying off of them. The harder he smacks the Petpets, the cleaner they become. When the janitor walks over the dirty spots, he cleans them up. On his circuit around the Hall, the Yurble will pick up water droplets, which add to a pail that the janitor can use to clean multiple Petpets from a distance. He can also upgrade mop heads, for more efficient Petpet scrubbing!