As an Echo, she wouldn't even receive any unique specials or recovery that'd allow her to use her ponytail. Otherwise she'd be elevated to "Melee Clone"-status that Pichu, Young Link and Dr.Mario are currently being like in Ultimate.
Right now her being an Echo would mean she'd use Bubble Gun Popgun and Rocketbarrel Boost, taking them from Diddy at large, while there'd be nothing new programmed for her as an attack, aside from a possibility of a different Final Smash compared to Diddy. That's how expendable her unique ability would become if she were turned to an Echo for Diddy.
Also, ninja'd by @ReeseBalaski, but they put it better and faster than I did, too busy reading replies updated here.
I never was against her being a semi-clone, or tried to deny her similar appearance to Diddy here, though. My point still is that making her be a glorified alternate costume for Diddy just because she's similar to Diddy in appearance and bodybuild, would not make any sense to what she's capable in canon, and wouldn't please her fans in general if she was received that way.