Don't forget Mew2. He sucks too, though not as bad as Pichu.
As for Marth, He's not considered the best character (he's getting really close though) but yes, he can be beaten. As with the other top tiers, you've gotta be able to play head games to take apart a Marth effectively. Though unlike the other top tiers, Marth has no counter character (Peach has Marth, Sheik has Falco and maybe Fox, Fox and Falco have Peach.) So yeah, I could see him knocking Sheik off her throne as top Smash character, but as I said above, no one's invincible in that game.