From the Super Mario Wiki
Mouse Trap is a microgame found in the Nature folder in WarioWare, Inc.: Mega Microgame$!.
There's a mouse that runs side to side. The objective of the game is to drop the trap dome when the mouse is directly under it.
- 1st level difficulty: The dome is very wide. It's 4/4 in diameter taking up nearly a third of the in-game screen's width.
- 2nd level difficulty: The dome is 3/4 in diameter.
- 3rd level difficulty: The dome is 2/4 or 1/2 in diameter, making it very hard to trap the mouse.