Horizon is very aggressively targeted at the audience of teenage girls and young women who are getting into gaming en masse.
It's about a beautiful post apocalyptic setting. it has a very Young Adult Literature plotline. It's rated T. It stars a young woman going out and confronting the greater world around her as she faces rejection at home. The weaponry focuses around outdoorsy tools. Most of the enemies are robots so you wouldn't feel bad about mauling animals to death.
The entire thing is built in the same mold as The Hunger Games, The 100, or Divergent.
I honestly don't think her clothing is put in there to sex up the game, but to give it a modern trendy aesthetic while maintaining the setting of the game.
There are a lot of games aiming toward this these days, and you can see it just looking through the examples listed in the thread.
You actually saw this a lot in the PS1 and PS2 era in regards to games starring guys. We can even just look at something like GTA 3 featuring an up and coming gangster trying to get into the world of crime. Similarly, a lot of the other leading games were about animals with attitude (who were generally teenage dudes) and JRPG dudes trying to save the world while all these stupid adults did nothing.
Now that the male audience is a lot older, GTA V was an ensemble cast of a middle aged guy with a pretty old family going through a midlife crisis, a nearly middle aged guy who had rejected the expectations society placed on him, and a somewhat younger guy getting into crime, but at an older age than you might otherwise expect. You will probably notice the sea of "Dad Simulators" in gaming toward that end as well, and the thematics very much being things targeting older male audiences.
Dishonored 2 and The Last of Us might actually be some of the best examples of both, with Emily and Ellie fitting the younger emerging female audience, while Corvo and Joel target the aging male audience.
But yeah, in general, while it's true that a lot of midriff type outfits are targeted at getting more dudes to play a game, I don't think that's actually the objective of this product.
I feel it partly has to do with aging game developers too.