Double post: because I'm worth it.
Jile, that's just the nature of the game. I can pick up King Dedede two hours before any of my local tournaments and plunder my entire region with reckless abandon. Meta Knight and Snake are also no contest. Besides the fact that "some characters are better than others," the fact that you perform this way is as much of a blessing as it may seem to be a curse.
Think about it. If you play a subpar character and do really well not just because you know the character well, but are a good player and know the game well, then wouldn't it be natural for you to be able to transfer those innate skills to a different character that's better and perform accordingly? It's just a testament to the fact that you can play the game, and shouldn't really be that frustrating to you. I know how you feel, trust me, but meditate on it some and I'm sure you'll find out what you want from playing Yoshi somewhere down the road.