While I agree that there should be more of an effort when it comes to alternative costumes, I wouldn't say it's for the same reasons. I'd like it if Olimar got alternative costumes for Louie, President, Charlie, and Brittany not because it'd mean we'd get a new female rep in the form of Brittany, but just having all those characters to chose from would be pretty cool. I'd like a Ms. Pac-man alt (echo would just be a waste IMO) not because she's a lady, but because it'd actually give Pac-man some not bare bones alt colors.
The devs shouldn't restrict themselves to fixing an arbitrary gender imbalance with costumes or characters and should rather just focus on adding more interesting characters in general. One can argue that by adding more female characters we'd see more diverse personalities but that isn't necessarily true, no personality type is owned solely by a single gender. Since it was mentioned earlier I'll use Tetra as an example. Tetra's got a cocky attitude and a general calm demeanor. She loves to tease, exudes confidence, and commands the respect of her crew. None of what I said is predicated on the fact that she's female, that personality can easily be attached to a male character and nothing would have to change (her role in WW however obviously would, but that's not what I'm referring to here). The opposite is true for many male characters is well since most of the time the character's gender isn't really an important factor of the character and was simply just picked for likely advertising reasons, or just because that's just how the character was envisioned. Basically, being a female character in Smash vs a male one doesn't really change much if at all, since moveset creativity is based more the game the character comes from than the personality they have for the most part, with personality only taking center stage when there's little source material in the game for the character to take after.
As well as this I just wanted to mention another potential defense to the OP's position, in that the idea that adding more female characters would attract more female players in general, diversifying the community and expanding the appeal to attract a wider audience. The main issue with this is that it assumes that women and girls will be more predisposed to a game that includes more women and girls when in reality we find that it has way more to do with the kind of game itself. Fighting games like Smash, at least in the competitive sense, are largely male-dominated due to the premise of beating the heck out of your opponent to attain victory, and thus no matter how much accommodation is made to include more female players without of course changing the game in it's entirety, you'll only attract so many players before the costs largely outweigh the gains.
Alongside this just like men, women are more likely to be interested in a character based on their personality first as opposed to sharing the same gender. From my personal experiences (so take that as you will) I've found that the girls I know rarely if ever pick female characters for their mains. Two of the most prevalent ones I know of main Dedede and Little Mac, largely because they love the characters and in the case of Little Mac, love his struggle. The Dedede main will sometimes play Peach because she likes Peach yes but it's more to do with her more girly personality in general than the gender similarities. The only person I know who mains solely female characters is actually a dude, and his reasoning is just that he enjoys beating the crap out of people with innocent, harmless looking characters like Peach, Daisy, and Jigglypuff.
Overall, adding female characters for the sake of them being female doesn't really do much of anything and the developers should instead focus more on making unique characters instead, whether they're male or female.