Only whiny parents will complain about this. Smash has been rated T since Melee anyways, and the only "partial nudity" in that game were things like Peach's panties "accidentally" showing up or Captain Falcon's skin-tight suit showing his chocolate bar abs and bum-bum (the later applies to Snake in Brawl too).
Samus wearing short shorts? This isn't even a bikini. Girls dress like this at my gym, some even like wearing this during the summer. The point is, they LIKE to wear clothes like this (hopefully none are being forced to do so). Whether it's to go to a party, work out, or just because they like the way they feel, that varies between.
Honestly, if it gets raised to T, I think it will be more because of the "fighting" game part. Solving problems through violence (however cartoonish) always raises issues with the ESRB.
Lool, since when does femenist = "I demand MALE chests!!!!" ? Maybe some of the demands are coming from femenists, but others could be from straight guys and girls trolling trollers in general, and female smashers or gay men who actually want to see them in lumber-jack positions.
I do agree though, these are actual costumes from the Metroid series, and the only other characters that would make sense to see them in beach clothes or something similar would be ones that already have been shown, like Robin and Lucina, from that Beach map DLC in FE: Awakening.