An article about this particular costume (link here) includes an interview with one of the costumes designers Aggie Rodgers.
... she remembered that Lucas pointed her in the direction of a
specific artist for inspiration, Frank Frazetta, whose illustrations
of fantasy and sci-fi characters like Conan the Barbarian and John
Carter of Mars have made him a genre legend. “He’s a wonderful
illustrator of the imagination,” Rodgers says. “I used to get books on
him all the time, and [Leia’s bikini] is very much from that milieu.
George specifically said he wanted a bikini, and so that popped up in
my mind, and we had the illustrator draw it up. I just kept pointing
to different illustrations that I liked, and we just kept doodling and
doodling and doodling until we came up with that.”
While this doesn't exclude the possibility of Carrie Fisher requesting a sexy outfit, it does indicate that George Lucas was after something fairly revealing.
The article goes on to how it was constructed in a way that wouldn't be overly or accidentally revealing given the amount of action that Fisher would be involved in.
According to Fisher (from an earlier interview), the costume was very uncomfortable.
[Fisher] recalled the physical discomfort that accompanied putting on
Leia’s bikini. “You don’t want to sit. There were creases, legs bent,
sweat. I don’t like sweating, I don’t know about you.”
Fisher was also quoted here as saying
That bikini was what supermodels will eventually wear in the seventh
ring of hell.
There are other comments from Fisher that are negative about the costume, but more from Leia's point of view.
When asked about rumours that Disney was removing all 'slave Leia' mechandise, she responded:
“I think it’s stupid,” she replied. “The father who flipped out about
it, 'What am I going to tell my kid about why she’s in that outfit?'
Tell them that a giant slug captured me and forced me to wear that
stupid outfit, and then I killed him because I didn’t like it. And
then I took it off. Backstage.”
While Fisher's views on the costume could have changed over the years, the only negative comments I can find (excluding the knock on effect of making her more of a sex symbol and maybe an accidental flash to Boba Fett) are about its construction and how uncomfortable it was during filming.