playing worse players can be harder for several reasons, but the major one is that no matter what the opponent does good or bad, you still have to deal with it. for example, a fox that just walks under you and upsmashes can still be a pain in the *** even if all they do is walk and C stick. there's other reasons too, such as a worse player is more likely to disrespect you on a poor choice (think fox up B directly at marth's fsmash) or take otherwise unnecessary risks.
when combined with a fast strong versatile character like fox, it's very possible to have a hard time with a less talented player.