Mario Kart Wii – How to Unlock Rosalina

There is no official information on what determines your rank. A good track time seems to matter the most, but many elements are speculated to also help:

Positively:

• lowest possible race finish time
• amount of time spent in 1st place
• boosts from mini-turbo, beginning of race, tricks
• successful item attacks
• successful wheelies

Negatively:

• falling off track or being slowed down by offroad areas
• hitting walls or obstacles

Though these are not confirmed, they serve as good guidelines.

Instructions

1. Identify cheaters early on in the game, if possible. Cheaters often have absurd win and loss records, i.e. 100 wins and no losses. They usually will have hacked the game to give them use of unlimited items. For example, if you are hit with a blue shell early in the race and no one has had time to even reach the first item box, you're probably dealing with a cheater.
2. Quit out of matches against cheaters. If you're able to identify a cheater before the match starts by looking at his win/loss record, then quit out before the match starts. If you quit out before the match starts you won't receive a loss. If the match has already started, quit out anyway. Attempting to race against cheaters at all rewards their behavior.
3. Report the problem to Nintendo. Although there isn't any official way to deal with cheating in Mario Kart, i.e. by banning the hackers, the Nintendo Tech Support Forum suggests that you fill out an online customer service form so Nintendo can investigate. Fill out all the pertinent information that you can remember about the match such as method of cheating and the player's name. A link to the form is available in the Resources section.

Source

And HERE is a link for a Nintento form.