It depends on the way you want to view it really.
The names behind Delphox and Braixen are both feminine in nature. Relating to Delphi (a female only clan of witches) and Vixen (Female Fox). However on the flip side, you've got the 90% male ratio. Which, if they actually cared about Delphox and Braixen being true to their name, they would've done something to make them 100% female or at the very least 50/50.
Then you have the appearance itself. Fennekin looks gender neutral, then you have Braixen who has a fur skirt and Delphox who has a fur dress. Though again, on the flip side, this can be seen as a robe of sorts, much like a mage such as Robin wears. Robin is also seen as both Male and Female in Smash, so I suppose the same could be said for Delphox.
The only two pokemon that are really male and female in smash are Pika and Jiggs respectively. Pikachu is clearly male because of it's tail and it wears all the male trainer hats, while Jigglypuff is generally mostly female and wears all the female trainer hats, implying that Jigglypuff is female in this game. However, with Greninja, Lucario, and Mewtwo, it's pretty hard to tell, same would go for Delphox.
I personally view Delphox as a male because I've got a Delphox character (the dude on my avatar) who is male. So I'm pretty used to using Male Pro-nouns for Delphox and even Braixen.