You can give any potential clone character an entirely unique moveset and say that they offer something unique. They could have done this with Lucina, or veterans like Toon Link, Falco and Luigi. What offering something unique to the game' really means is that a character has gameplay mechanics in their games that are different to what is already on offer in Smash.
The reason why these characters are clones or semi-clones (apart from just saving development time) is because they can't be made too unique without heavily deviating from how they play in their games. This is because they're very similar to who they've been cloned off in their games. For example, Toon Link could have wielded the skull hammer from Wind Waker instead of using his sword and prrojectiles. This would make him play completely differently, not only to adult Link but to the rest of the cast. The reason why they didn't do something like is because it wouldn't be reflective of how he played in his games.
Dixie is similar. Her only distinction from Diddy is her hair, which only manifests in uniqueness in one technique- the ponytail twirl. Apart from this she basically functions exactly the same mehanically in their main games apart from being slower. In DKC2 they were so homogenous that character selection was irrelevant for completing the main game, it only mattered for bonus content. The problem for me is that the twirl has been taken by DK's upb, so I don't see what she has in the main games that would be unique in Smash. The thing that made her hair unique was the twirl, other than that it's the equivalent of Diddy's tail. I could see them making her an alternate skin, and just using her hair for all of Diddy's tail attacks.
At the end of the day, the reason why people suspect Dixie as a clone is because she's similar. It's why people suspected characters like Luigi and Toon Link and Luigi to be clones too. No one ever though Diddy might b a clone of DK because they're not similar, just like no one ever thought Zelda would be a clone of Link, but suspected that Toon Link would.