-liberation movement in the early sixties help confirm this?
The Female Uniform seen in most of Star Trek's original 3 seasons was not, so far as we know, a Roddenberry creation or idea.
For Roddenberry's concept for uniforms on the series, you need to look at what was worn in "The Cage", the original Pilot for Star Trek. In that, all crewpersons wore the same basic uniform, regardless of gender. This continues into the 2nd pilot, "Where No Man...," where the redesigned uniforms are still worn by all genders.
In between the production gap between the 2nd Pilot and start of series, the new regular, Grace Lee Whitney, recommends an alternative uniform. The Production Team adopt her idea, and thus the well-known Female Duty Uniform is introduced, although you still see background Women characters in the OG uniform for a couple more episodes ("The Man Trap" and "Charlie X")
According to Whitney, Roddenberry did the final sign off on the uniform design, with her as model. However, she implies it's at the end of a process she went thru with Theiss, the costuming head for the show, where she wanted an outfit that "show[ed] off my assets." One can assume it was then aligned to the other women crewmembers on Star Trek.
As for "why," aside from the design being "eye-catching," I've never seen a political/social reason given as a memo or other primary source for the choice. The kinds of references /opinionswe do know about are sampled by Shatner, who is quoted as saying at the time it meant that Trek could "boast the shortest skirts on women of any regular series on television."
But no, no contemporary references I know of to sexual/women's liberation, around this design. That is not to say some opinions on "why" weren't stated after the fact, as Trek grew in popularity -- however that's a separate, and more involved, conversation.
Two references: