In my opinion:
Very High learning curve: Sheik, Yoshi, Ice Climbers, Snake, Sonic, , Zero Suit Samus, Diddy Kong
High learning curve: Bowser, Peach, Ganondorf, Pikachu, Samus, Lucario, Wario
Average learning curve: Mario, Luigi, Donkey Kong, Link, Zelda, King Dedede, Fox, Captain Falcon, Squirtle, Ivysaur, Charizard, Jigglypuff, Ike, Ness, Lucas
Low learning curve: Toon Link, Pit, ROB, Falco, Wolf, Kirby, Olimar, Marth, Mr. Game and Watch
Very low learning curve: Metaknight
If I were to set up a graph showing this, it would look something like this:
VHLC>>HLC>ALC>LLC>>VLLC
To explain each:
VHLC: Playing as these characters straight forward will pretty much result in you getting the crap kicked out of you. A lot of thought and practice has to be put in to be good with these characters and there's an enormous differences between those that know how to play them right and those that don't. Usually involve learning a decent number of techs, strategies, and proper spacing to really excel with them.
HLC: Take a lot of thought and practice but not quite to the degree as the those with a VHLC. Also, you can still be successful with them even if you aren't completely familar with them but it's not all that likely. These characters have their techs too but they aren't completely essential for a win usually (like with Pikachu's QAC).
ALC: Like the name suggests, average. They have their minor quirks but they usually aren't too hard to pick up. Usually you just have to be familar with the characters' strengths and weaknesses to use them with a decent amount of competence.
LLC: These are characters that you can get familar with within a few matches or simply by watching videos. Many of the strategies used by these characters are universal too and not overly complex which makes them even easier to pick up. As you may have noticed, most of them are on the high tier spectrum on the board too which would explain why a lot of them are popular secondaries.
VLLC: Count the number of fingers you have. Are there ten of them? Can you move them? Now, can you hold a controller? Good, then you're good to go.
I'm somewhat kidding but Metaknight has a lot more options than most characters thanks to his awesome offstage game, high priority attacks, lack of lag for a majority of his attacks, and his 5 methods of recovery. It also helps that he can almost quite literally shut down nearly half of the cast with some of the things mentioned above. Switching from another character to Metaknight is almost literally a day process.