Most people, probably. But I'm one of those (few?) people who do care about gender in videogames. And to me it doesn't matter how awesome a character plays in Smash Bros, if they're not female, I won't play as them. Maybe I'm narrow-minded, but if I have a choice between a guy or girl, I pick the girl every time. The characters I play in Smash are all female, even though I personally like the way some of the male fighters play as a lot better, but because they're male, I won't play as them. So gender does matter quite a lot in games.
That is your choice, but honestly, you're just holding yourself back that way.
You could be missing out on fun characters in Smash that have a more fitting gameplay style for you and miss out on whole games because they don't have female MCs.
It's like playing a Metroid game because Samus is a girl but not enjoying 2D platformers at all.
Full disclosure here: the main reason I got into Smash was because it was a hype inducing crossover of various games and had both Mario and Pokémon characters, who were the only characters I could even recognize out of the entire roster, and it got me into various Nintendo franchises.
But reason why I still play Smash is because it's such a fun and unique game in terms of gameplay and style. There's hardly anything like it in the industry and many have tried to replicate the success of this franchise to no results.
I'll admit, wanting a character because of their gender or sexual orientation isn't too different than wanting a character because "we need more villains" or "we need more brunettes". That being said, none of these things are good grounds for supporting a character.
I get that people like getting characters they can identify with, but simple things like this should have little to no impact on who gets chosen. Look at the character in terms of moveset potential and iconicness, not representation of a specific group.
This a thousand times.
The only difference is that gender and sexual orientation bring up all the politics alongside it, so it makes discussion automatically more toxic and vitriolic, but it's the same ****.
Do characters in Smash have some big neon light saying how well-written they are? No.
Corrin comes from a ****ty game with poorly written characters. Still a fun character to play as in Smash. Your point is null and void.
Also people didn't know jack**** about Fire Emblem back when Melee released but they still liked the characters from that game despite going wtf at a first glance, but then played as them and were fun to play as.
So, my point stands. Cool characters with cool abilities and, most importantly, fun gameplay matters the most. Marth is one of the top tiers in Melee for a reason. He's well designed, viable and fun to play as.
tl;dr - I don't give a **** about representation because that shouldn't be the deciding factor in which characters get in the game. I'd rather have some obscure "wtf is this" character that is fun to play as than some "OMG LGBT representative, how progressive" kind of character that has a boring and not fun gameplay design.
I'm done with this discussion.