They didn't put it in because it wouldn't make sense for Samus to start stripping on her ship while watching the Chozo temple explode. The helmet was enough for me.
And fit girls don't have to be masculine. You know all those beautiful actresses and supermodels. They work out a lot to look like that. I imagine they're quite strong, so Samus doesn't necessarily have to look manly to be strong.
Yah, and Chozo are bird people, so I imagine that'd make her even sleeker, if it changed her appearance at all.