On mario kart DS, when u play it on the R4 card, there are lots of cheats you can use, like Always Have Item cheats or Have Trophy cheats or 3 Star Rank cheats and many more! Never tried them out on multiplayer or WiFi though.
I don't own any Mario Cart games, but I own Metroid Prime Hunters for DS and I hate hackers so much >.< They knock me down in ranks and it's no fun when you get matched up again hackers everytime.