Any character can beat any character, but at high-top level play, we will see (and have seen from other fighting games) less and less of that. Sheik, ZSS, pika, wario, rosa, sonic, fox, diddy, and more will start to break away from the pack at that level, and will be MUCH MUCH harder for people who main Duck Hunt, kirby, D3, DK, ganondorf, Ike, Marth, mew2, WFT, zelda, doc mario, -and such to outperform top tier characters.
EDIT: oh, and people play DK becuz he's fun for them. Of course, those who are dedicated enough with him will win games and maybe even take sets off of players at nationals. But, as the meta progresses, we will see fewer of these players since the skill gap will have to become noticeable and at top level player, there really won't be any noticeable skill gap.