Gender, sexual orientation, race, etc., are characteristics. However, they should not be THE defining characteristic, especially creating well-written characters (but these aspects can certainly help with development/depth). Unfortunately, there are exceptions. I've seen too many cases where these characteristics ARE taken as the defining trait, and most of them lead to stereotypes (or perhaps the [flat] "token" [insert gender/sexual orientation/race/etc. here]... but I won't get into that). I like to remember characters for their good character development, growth, how well I could empathize with them, and especially learning morals and lessons from them regardless if my own gender, sexuality, etc. is different... among other things. But I don't want to make a huge argument on what makes a "well-written" character -- I think people have different definitions anyway...
Well, that's just my two cents on the whole thing. Applying this to Smash: being female shouldn't be the deciding factor. Otherwise, I personally think it's a shallow reason to include such character. Not to mention Smash itself is a different context altogether; there are other factors to include as well, just like any other character, really. However, in the end there are some exceptions, making it hard to know how the heck Sakurai chooses things anyway.
Tbh, I haven't really thought of the question "does Smash need more girls?"a lot. Perhaps because I'm more concerned on video games in general including more strong female protags/roles.