Opponents
If you find your opponents are too quick, try re-playing an older track to build up your funds. Once you have enough cash, either upgrade your existing mower or purchase a newer, faster model.
Tomatoes
Although it is tempting to pelt your competitors with tomatoes, it is not worth the effort. Just concentrate on getting your mower around the track as efficiently as possible, and you will not need to take aim at your rivals because they will be eating your dust.