Sour Supreme
サイマグネット
You should post this in the Female Newcomer discussion if you haven't. I touched on this idea, although it was brief. The thread could use the discourse.I think the problem with getting more women and racially diversifying the cast lies less with the idea of it, and more to do with the theoretical execution of it based on the current breakdown of the games industry.
I apologize in advance for the length. However, given the sensitive nature of this topic, I wanted to be as precise in both my speech and logic as possible when sharing my thoughts on the matter.
Counting Sheik as female, the current gender ratio for the roster is approximately 47 males to 12 females (not counting characters who have alternate gendered costumes like Robin or Pokemon Trainer, genderless characters like Dark Samus or R.O.B., characters who represent a species rather than a specific individual like the Pokemon or Piranha Plant, or the Ice Climbers and Banjo & Kazooie who have a member of each gender).
Likewise, nearly the entire cast is white, east asian, or ambiguously between those two, with the only characters not falling into those camps are Pokemon Trainer, Villager (who both have alts of multiple races), and Ganondorf (who is debatable, given how he's technically a fantasy race).
So, it can be seen that Smash is pretty homogenous in terms of race and gender. However, the question then becomes, what do you do about that? The obvious answer is put more females and more racially diverse people into the cast. That's a lot easier said than done, honestly.
Most of Nintendo's leading ladies are already in the game, such as Peach, Daisy, Samus, Lucina, and Isabelle. Off the top of my head, the only remaining female Nintendo characters I really think have much of a chance at this point are Ayumi Tachibana, Style Savvy Girl, and Edelgard. Likewise, there aren't that many third party franchises with female leads, so that's a pretty dead end as well. Out of all the non-Nintendo games I can think of, the only major female leads I can really think of Jill Valentine (unless you count Chris or Leon as the lead of Resident Evil), Lara Croft, Shantae, Nakoruru (unless you count Haohmaru as the lead of Samurai Showdown), and Kasumi (unless you count Ryu Hayabusa as the lead of Dead or Alive).
Games with non-white/asian protagonists are even harder to come by. The only one I can think of is CJ from Grand Theft Auto. Likewise, the only Nintendo character period I can think of that fits that description is Anthony Higgs from Metroid: Other M.
So, then the question becomes, do you give characters in these categories priority over other candidates who may be more popular or could translate themselves in a more interesting way? Ultimately, that's a subjective judgement based on personal values. However, I don't think its as easy as simply pressing the "add underrepresented groups to Smash button", given how the imbalance between these groups will still be relatively vast even if Sakurai were to go out of his way to add characters to solve this issue.
Likewise, I think it's the fact these groups don't have an abundance of candidates that makes the Smash community less supportive of them as opposed to a quota like "villains" or "side characters". I don't think the movement to get villains into Smash came about because people really liked the idea of playing as the bad guys, but instead came because a handful of villains were really popular and people wanted to play as those characters. That's why a lot of people stopped playing the "Smash needs more villains!" quota once Ridley and K. Rool joined the roster. There wasn't this great sentiment that Smash finally had enough bad guys, a lot of people just supported it as a vehicle for those two in particular and withdrew there support once they got what they wanted. Likewise, the side character quota didn't really come up until recently, when it was felt that characters like Waluigi and Bandana Waddle Dee had a realistic shot now that most franchises have their main cast in order. Put simply, I don't think much of the Smash fanbase will support the "female" and "racial diversity" quotas until they have some popular faces who fall under those labels.
TL;DR: There is a gender and racial imbalance in Smash Bros. However, the issue lies in the the lack of prominent females and non-white/asian people that Smash could really draw from. Likewise, most of the Smash Bros. community isn't super supportive of the idea given this lack of prominent candidates to draw from.