I know this post is from a while ago, but I would say Sheik is his worst matchup. There are two big reasons, the first being needle camping and the second being that Sheik is the best character in the game. However, besides Sheik, I'm a firm believer that the matchups for Little Mac are extremely dependent on the stage. For example, on a stage like Smashville Mario is his worst matchup because once he gets the percent lead, he can just get on the top platform and camp with fireballs. The only real way to approach him while he's doing this is to Up B, and he will not only see it coming but it is also extremely punishable. Sheik is also a horrible matchup on Smashville, but this is just because it's Sheik on Smashville. Another example would be that on FD, I believe his worst matchup is Sheik and Lucas. Sheik because of reasons mentioned previously, and Lucas because he can essentially perform an impenetrable wall with his PK fires and that snake thing. Similar to Mario, this makes it almost impossible for Little Mac to approach.