They put her in that costume to try and make her hotter.
(1)this demonstrates that they had no understanding of what an attractive woman should look like as she'd have been much more attractive in a starfleet uniform with her hair down than a tin foil catsuit and headmistress' hairstyle.
they succeeded in making her look frumpy and about 10 years older
They did the same thing with T'Pol on ENT and pretty much the same thing with Deanna Troi on TNG for a few seasons.
I think all the catsuits spanning these series illustrated (1)