That's absurd, some characters are more definitely harder to play well than others. Look at Sheik vs Yoshi for example. Sheik is a character that almost anyone can pick up and play decently, as all of her moves string together pretty naturally. Most of her aerials autocancel, and she has a lot of easy set ups and into kill moves. On the other hand Yoshi is incredibly easy to punish, and he's very slow and clunky without a lot of experience. There's definitely a difference in the ease of access to different characters in the cast.
To answer the OP's question, I'd say Sheik is the easiest to pick up because of the reasons I stated above. Marth is probably a close second, but he's a lot harder to bridge the gap into mid-level play than Sheik imo, because he's punished really hard off of simple mistakes or when he tries to over-commit.
Fox is arguably the best character in the game, but I think he's far from the easiest. Fox relies heavily on using his speed to overwhelm the opponent, and he's easy to lose control of because of that. And being a fast-faller, he's combo bait for pretty much any character.