I still firmly believe that Dixie Kong not being included as an Echo Fighter in Smash Ultimate's base game is a sign that yes, the developers see her as more than just "girl Diddy". If that's all she was to them, she would have gotten in already.
The only semi-clone in the base game was Isabelle, and I think this is 100% understandable. Isabelle has just been a steadily rising star and her absence as a playable character would have been weird to Animal Crossing fans, but her proportions don't match Villager at all so when making her a clone, they made her especially unique and even gave her the fishing rod that some Animal Crossing fans were confused wasn't in Villager's moveset.
Also, and I hope DKC fans don't take this too personally, but in the time around Smash 4 and between 4 and Ultimate's release, Isabelle was simply in more games than Dixie was; Isabelle was always going to be higher priority for that alone if there was only one clone they could upgrade to semi-clone, or only one spot for a semi-clone in general. The developers needed to maximize the amount of full blown newcomers they could implement with the limited newcomer space they had with Everyone Is Here, and even Echo Fighters take time, and I think every base game Ultimate newcomer was chosen very well (Incineroar could have been any Pokémon Sun and Moon creature, though. Pokémon is probably the only series to have an actual "slot" for a fighter these days).
I think if Dixie ever gets in, she'd be a semi-clone like Isabelle was, or at the very least a Chrom style Echo Fighter where her up-B is different. Of course, like I stated in the first paragraph, they could have just done that in the base game but they didn't, so I'm more confident in the semi-clone theory. Here's hoping again that the post Pass DLC will indeed be a mix of third-party and first-party (I don't see Sakurai or Nintendo stopping the third-party train) and that Dixie is part of the latter even if the latter is partly advertisement picks from recent or upcoming Switch games.