MKWii may seem a bit more "noob friendly," but if you're put in a room against experts you'd still get slaughtered. The same is true for most games - however, there are fewer people on MKWii that are as skilled, and it has a rating system in order to try to keep the games close.
That's all it takes, really - a decent online that can implement a good rating system. The game doesn't need to be dumbed down for the masses - all it needs to do is pair you up with others that are on a similar skill level. Gears of War 2 tried this, though they sort of failed at it. I'm confident it'll get better in time, though.
edit: My conclusion is, while Nintendo's games may be a bit more friendly than others, the main difference comes in the people you're playing. You may just be playing more noobs in MKWii, due to the demographics playing and/or the rating system they implemented.
