It was Ron Moore's idea to put Troi in the uniform, which Marina Sirtis was eager to oblige with.
Moore himself has commented how ridiculous Seven's catsuit was, calling it "eye candy with no content".
LOL, did he say it with a straight face?
Here are Moore's comments about VOY:
Skip to "PART IV: RON MOORE WORKS ON STAR TREK: VOYAGER"
section to get to the catsuit comments.