^^^
I can agree with that. It's obviously much more difficult to find and exploit advantageous situations for certain characters who aren't as well endowed such as Captain Falcon in comparison to Metaknight. In this sense, sure the character could be said to have a significantly steeper learning curve. More time is needed to understand their moves intimately because they are more situational and can't just be spammed like Metaknight's neutral B.
However, when you compare it to the learning curve of some characters in Melee, it's really almost funny to say that any Brawl character has a significantly steep learning curve. Learning to play Ness exceptionally well in Melee was exponentially more difficult than learning Ness in Brawl. I spent tons of time in Melee learning how to effectively control Fox (waveshine, drillshine, multishine, shine->Bair, etc.) while in Brawl some of the most complicated advanced techniques (supposedly) such as Sheik's DACUS and the Ice Climbers' infinite chaingrabs took only a few days to be able to do consistently. The only things that are truly difficult to master in Brawl are the simple things like how to space certain moves, which moves to punish with, understanding the hitbox size of various moves etc. These are things that were just as necesary in Melee but were accompanied by a vast assortment of advanced techniques which were difficult to perfect as well.
So yeah you can say that pretty much every character in Brawl has a steeper learning curve than Metaknight. But it's really not by all that much. An excellent player will be able to learn his or her character very well in a short time either way and their ability to win will be based on the calibur of player they are and the character matchups more than "well that character still has potential to be unlocked." If this could be the case at all, it is very shortly going to come to an end. I can see how it could be argued for characters such as Pikachu in the case of Anther but let's face it. It won't be all that long until a character is what it is and nothing more. The trick won't be "learning" a character but rather knowing him/her backward and forwards as any good player should for their main(s.)