My two cents:
If you count peeps who are not confirmed yet, I think she is definitely a top 10 here or outside of the forum. Random Pokémon #850, Waluigis, Ashleys, random Xenoblade 2 whatshisname and stuff aren't as popular as her. (The only VERY popular Pokémon not playable is Eevee and it is once again in a Pokéball).
There are definitely folks more popular than her who aren't playable or confirmed yet, such as Geno and probably Isaac, but I can't think of ten or twenty potential fighters that outshine her in popularity.
In my opinion, you are underestimating Dixie's popularity.
It's true that most of her competition has either hit the big time or been sent to the graveyard. She was undoubtedly losing to Ridley, Wolf, Rex/Pyra, K Rool, Bandana Dee, Ice Climbers, Snake, Inkling, Isaac, Ashley and possibly Geno before the game's reveal, but a large number of those characters are no longer in contention.
Ultimately, it would take an unrealistic amount of work to deduce her exact standing in the overall pantheon, as there are millions of fans to consider who use different media to talk about Smash, and many are from other nations. I've seen her not even make top 25 in one poll, barely make top 20 in another, yet still hit top 10 or even top 5 elsewhere. She's probably one of the most inconsistent characters in terms of popularity, as she's all over the spectrum in different places. This is all I meant when I advised we use caution. I don't want us being shocked and appalled if she doesn't make it in but characters we see as "less popular" do.
It's always good to not lose sight of the bigger picture. Remember Waluigi and Ashley. We thought they had everything working in their favor and they were big competition for Dixie. And well...look at them now. It leaves us to wonder if they were as popular as we believed, or if they were just considered non-priorities in this massive game.
All that said, Dixie has a couple things going for her that they didn't:
-Echo fighter potential because of her resemblance to Diddy and relationship to him.
-A return to prominent roles in a recent game that has yet to show up in Smash despite it's success.
-Her series is getting recognized this time around to a much bigger degree.
-She actually jumps, runs and fights in her games. Nothing needs to be compromised, re-imagined or re-invented to make her a fighter. You could lift 50% of her straight from her games and she'd fit in just fine.
Don't think me defeatist. I think her odds are damn good. I just don't want us to think shes a lock just because our awesome community is all on her side.