This isn't for how easy a character is to pick up and play, it's how difficult they are to OPTIMIZE. See explanations below.
SG: His basic playstyle and his optimal one are practically identical. Braindead character. RC: Can struggle against some characters in neutral, but mostly just a basic character with too many trump cards Puffy: Easy ToDs, ignorant moves, and a mostly aerial gameplan make puffy easy. Psykid: It may be controversial that I put him here, but he is not that hard. His ToD is easy, magnet is not really strictly necessary, and he isn't really too hard. If I get enough comments, I'll bump him up a little. CE: Harnessing his speed is hard, everything else isn't. Chrarth: Most balanced, honest character in the game. Unfortunately, he's too honest for his own good. Spacedog: I used to think he was harder. After giving him a whirl, he's not all that. Cat: He's playing his own game. His game plan revolves around evasion, essentially playing Tekken whilst the others play SSBR. Mike: Requires lightning fast reactions to DI to consistently land his strongest combos. Also pretty evasive. Mariano: Second hardest ToD in the game. Unforgiving execution makes him hard to play at the highest level (remember this is an OPTIMIZATION tier list, don't bring up fireball spam). Pakku-Man: Trash character smh, difficult neutral. Like trying to play Rachel Alucard in SSBR, but not nearly as strong. Metron: Hardest ToD and most unforgiving execution in the game. No room for error. CBs: The reason we collectively sleep on this character is because desyncs are difficult to utilize optimally. Stuff like optimal shield pressure setups haven't even been discovered yet. Relatively straightforward ToD though that works on everyone, however.