The "Princess Brand" is purely a marketing tool. Admittedly so by its creators.
And yes, including Mulan made no sense with regard to the purported purpose of the brand, that is, to highlight Disney princesses. But it made marketing sense since Mulan couldn't, by herself, keep her merchandising franchise going solo. And so, she joins the other princesses, who, together, are synergetically more than the sum of the individuals.
And it made sense to be intentionally diverse... so that they can be merchandised to a diverse audience. This isn't virtue signalling. It's capitalism.
But the whole things hurts the brain to see how the brand is handled. Moana was the last one officially added, but with no 'induction ceremony.' Anna and Else were never added, but they were their own merchandising/franchising juggernaut (in retrospect, they could have left Moana out for the same reason). Originally, Tinkerbell and Esmerelda were part of the line-up -- a sure sign that 'princess' was originally meant in the loosest sense -- but they got dropped quickly.
Sofia and Elena never got inducted, maybe that's because before the age of Disney+, TV still had that lowbrow stench compared to cinema. However, Elena now regularly appears with the official princesses on floats and M&Gs.
Raya, as a chieftain's daughter is technically a princess in the way Moana or Pocahontas is, but I haven't seen her hobnob with the branded royalty... yet.
Mirabel is the granddaughter of a sort-of Mayor of a town, and maybe the magically-chosen successor to her grandmother... but with Mulan in the mix... does pedigree matter anyway? Besides, it seem that Mirabel/Encanto doesn't need the Princess Brand to carry her own merchandising weight. And with Elena unofficially in the mix, the Brand already has a Latina Princessa.