In order to reconnect with her humanity, Seven had to return mentally to her last wishes and desires as a human child, Annika (Chakotay learned her human name via a neural transceiver in Scorpion.) One of those wishes (as Seven admits in One Small Step) was to be a ballerina. The leotard helped her to connect with this memory, and the high heels helped with muscle memory of pointe technique. She might have had a tutu except that it would get crumpled in her regeneration alcove and impair her ability to use the waist-high control panels all over the ship. So Seven sauntering around Voyager like she did was absolutely essential to her recovery and not gratuitous or exploitative at all. QED.