Sure costumes can deconfirm characters. Daisy for instance. She's pretty much deconfirmed. =/
And heck, the Dark Fox proves just how close Wolf is. I'd say it was a really bad blow to Wolf's chances, but as Wolf's apperance was always like that, the costume really doesn't change anything we didn't know.
Since when was Daisy ever considered for Brawl in the first place? She and WaLuigi are in the same boat. A costume does not deconfirm them, but the fact that....
1.) The Mario series has more important characters that Daisy that should appear in Brawl, such as Bowser Jr. for example or even Toad before her. Daisy is simply out-prioritized.
2.) WaLuigi will not appear as a PC in Brawl because, unlike Wario, he never starred in his own game and he is simply a filler character to Mario Party, Mario Sports and Mario racing games. If WaLuigi never existed, the Mario and Wario series would not be effected. Other than the Mario Bros. vs. th Wario Bros., WaLuigi sucks.
3.) There is already enough characters for the Mario series that Daisy and WaLuigi could not even be considered. There are probably going to be atleast 5 roster spots for the Mario characters so Daisy would not make it. Its not a costume that deconfirms her, it is her insignifcance to the series.
4.) The fact that Mario has a Wario costume and Wario is still in the game. If a costume deconfirms a character it should be a principle that applies in every case.
Fox's costume does not deconfirm Wolf because...
1.) We only have 1 SF rep, Fox. The other 2 spots, assuming there are 3 spots total, are up for grabs at this moment. Hardly any characters are worth putting in Brawl outside of Falco, Wolf and Krystal. Slippy and Andross are Assist Trophies so they cannot be playable.
2.) There is a need for villains/rivals/antagonists in Brawl. So far we have Wario, Bowser, King Dedede and I guess MetaKnight, since he is a rival to Kirby. Assuming Ganondorf returns and since King K. Rool and Ridley are not confirmed, Wolf would make an excellent choice for a rival/villain character. He is Fox's main rival and would make sense since Andross is deconfirmed because of the fact he is an Assist Trophy and he is the only real significant bad guy left in the Star Fox series since characters like General Scales only appear in 1 game.
3.) Wolf actually has demand to be in Brawl, WaLuigi and Daisy do not. Look at how many fans on this forum alone call for Wolf, where as there are not as many for WaLuigi or Daisy.
I don't think Wolf is deconfirmed as a character at all and I don't see how a costume hurts any characters chances.