You're entitled to opinion, but others can weigh its value.
What you say is in a sense true, every character can be played equally well, in the sense that every character has some sort of maximum potential to be brought out (whether its been fully discovered or not is another subject)....
...however...
...Your opinion is uneducated. Fighting games are like a game of chess, an intellectual battle field when taken seriously. Though obviously there is a key difference. Picking a specific character is analogous to changing the number and type of pieces each player would have in a chess match. Its like having one player start with three bishops and one less rook (Specifically placed of course) and the other having four rooks and no knights (or picking a different character.)
A fighting game like brawl where each character has so many more properties in a single attack then each piece individually has in chess is impossible to balance perfectly. A lot of times having 4 rooks is better then three bishops...
Take a game where there a 20 characters that are all exactly the same except for one whose attacks all do an extra hp/percent of damage. That character would be instantly better then all the others, even if slightly so. Even if you try to balance it out by say, making his attack slightly slower, you would either make that character worse or better then all the others. If your changing the characteristics of characters there will be imbalance.
The only way for a fighting game to be perfectly balanced would be if all the characters were exactly the same, and that game would never sell and be boring as hell. Its basic design. The best thing they can do is minimize that imbalance will still making the game diverse and otherwise fun.
Like I said, you're entitled to an opinion. However, when opinions stray into the realm of facts, they lose.
Fight a DeDeDe worth his salt using DK, see what happens, and get back to us.