Personally, I think that the clothes should fit the character, the role, and the universe.
Character: Morrigan would have no business regularly wandering around in Chantry robes.

Role: No one who frequently gets into firefights and is planning on going into a suicide mission should be wearing a leather strip as an impromptu bra. (Looking at you, Jack!

)
Universe: Merill should not be wearing a spacesuit, and Wrex should not be wearing full plate mail. (Although that would be incredibly awesome

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FURTHER EXAMPLES.:
I think Isabella's outfit was fine, because a pirate whose role is to be agile needs to not be hampered by wearing a whole lot. Granted she took this pretty far, but pants are overrated, so I'll let it slide.
Morrigan's outfit was fine, because if everyone was doing their job, she should never have ended up in melee combat anyway. Also she was raised in the woods, so what clothing she did have was likely scavenged and thus understandably threadbare.
I'm not really sure why Samara bothered showing so much cleavage, there was certainly nothing that I saw in her character that would justify her using her attractiveness offensively.
Miranda's outfit had a bullet-resistant weave or whatever in it, so it granted protection. Not as much as full armor, but she (a) is a caster/techie. and thus should be avoiding the heaviest fire anyway (

would definately use her attractiveness as a weapon, so making it snug in all the right places is justified, IMO.