I think "slut" is a little too far. It's not like she was wrapping her legs around all the men she came into contact with.

I think in FFX, she was trying to be the girl everyone wanted her to be. She had a lot to live up to because her father was a summoner who defeated Sin. I also believe she knew or thought that defeating Sin meant deleting her life from the world. It's a lot to carry on your shoulders. I don't even think her friends all helped that much either. Wakka and Lulu were always arguing, Kimahri was always so quiet and very protective, and Tidus just constantly made a fool of himself. I'd not feel that comfortable around them.

In FFX-2, she had much more freedom. She didn't have a weight on her shoulder like before, and had a job with the Gullwings and hunted spheres in the hope it'd bring her Tidus back.

The entire game was very girly. Girly doesn't always mean slutty though.