Echo Dixie only makes sense if you've never played DKC for more than a couple of minutes and also if you ignore basic Smash visual design elements.
First of all, having this giant yellow appendage that gets in the way of all her animations but never has a hitbox nor a hurtbox would be very confusing and feel pretty amateurish. Echo fighters can cut some corners with their portrayal (like Daisy having the blue Toad or Dark Samus lacking her unique abilities) but not if it messes with the readability of the action. It's just not the kind of stuff you see in an official Smash game. It would be a mess.
Then, while you could replace the Peanut Popgun with her Bubble Gun and have them work mostly the same, having her use the Rocketbarrel Pack would just feel wrong, as she 1. never used it, 2. is capable of recovering without it, and 3. it doesn't really fit with her design given how her back is already occupied by the ponytail.
Plus she never does the cartwheel, which appears in two of Diddy's normals. Coming from a platforming series, they already have built-in stat differences that kinda have to be preserved in what is still in part a platforming context: she runs and attacks slower than Diddy does and she can't jump as high as Diddy, but in exchange she can float and move almost horizontally through the air.
Lastly, she's never shown to be as acrobatic as Diddy, while she does have lots of canon abilities that Diddy doesn't have. Including a Dixie that doesn't use her ponytail is like including Banjo without Kazooie. What's the point?
Echo Dixie is a no no.
Wolf/Lucas/Luigi style semi-clone could work. But at that point you have different stats, different normals and similarly-inspired but still different specials, which is basically a unique character...