I've come to voice my support for Dixie!
As for the topic at hand, Dixie has an advantage in her design that most potential newcomers don't have. She's the same size as Diddy, but her many differences from him mean there's no way she can be a complete clone. Lucina and Marth have the same abilities, are the same size, use the same weapon, and are both humans, so they can be closer in their respective movesets. Almost all of Dixie's abilities differ from Diddy, and let's be real, you'd have to be an idiot to put Dixie in a game and not integrate her ponytail in any way.