Yeah, it sucks how the diversification of characters backfired so badly.
Nintendo tried to make each character unique, but they did too good a job of that and now the only way to win consistently is to pick those combinations.
I wish they took the MKDS approach. In MKDS (on wi-fi) all of the characters are perfectly equal and can fit in any kart (or bike, MKWii didn't introduce bikes, MKDS had two) they want, and you can't get pushed by anyone so you don't have to worry about getting "lag-smashed" off the track. The downside is you need an AR to play on all of the tracks and items can't be dragged. (The former was due to moving objects, such as cars and chomps, even though it works pretty much normally, save a few errors in detecting your rivals getting hit which have no effect on gameplay, and the latter is due to them trying to prevent dragging lag.) MKDS also let you chose which vehicle you want AFTER you find out who your opponent is, so if they have a really bad rating, you can choose a larger variety of karts, and swap vehicles again after four races. In MKWii you're stuck with the first vehicle you chose for the whole wifi session, so you HAVE to use your best bike if you don't want your rating to go down, which odds are it will due to the stupid items.