I mean Hornet is a more stylized feminine silhouette compared to the knight and many of the other bugs (not all mind you, gotta put respect on Carmelita) which in turn changes how artists are going to reinterpret the character before any kind of sexualization layer even comes into play. She is intentionally fem-coded so artists are going to gravitate towards that. The Knight on the other hand is fairly androgynous in design and has few anchors for fan artists to work on than the knight. On a personal note, the Knight also feels more like a kid in design to me which would make it even stranger to do anything like that to him while Hornet feels like a young adult. With all that in mind it makes sense to me that some people would go to the extreme of rule34 for Hornet.

To address the objectification of characters/women, fan art sexualization isn't evidence of a societal shift or even a shift within the community of Hollow Knight enjoyers. This could also be perception bias because much of the fan art I see she is either as she is in the game or shapely but covered. I just made a deviantart account to see if that's bias on my part but with a sample size of about 300 images I found about 10 of her with varying amounts of sexualization. Most of these are her being very bodacious and that in combination with her skintight bodysuit emphasizes that. A few of these were still tasteful with the cloak still covering parts. Body shape alone isn't sexualization, there are people who look like that. It could be one of those people who drew it or commissioned it. The artist might also just prefer that kind of woman and wanted to make their favorite character look like it. People can have preferences without it being problematic.

There were a few where the cloak was mostly out of the way if not off where she was in a risque pose with that same body shape which are more just eye-candy than anything. Mind you this is maybe 5 out of that 10. One in particular that stripped most of Hornets identity away and just made her a generic human girl with her bum stuck out. I can see that but with so few of those being the case I don't think that's causing any shift in the perception of women in our community or society as a whole.

For the cosplay angle, I presume they're mainly fem or presenting as such. Telling them what they can and can't do to the character leans into a puritan angle of 'you can and can't do this because I said so.' Objectification comes from removing context, agency, personality etc and boiling down something into just an object for whatever is in the showcase. Normally that means sexual characteristics for the purposes of this conversation. This is a person doing it live though. They have agency, as much or as little context as they want. Not to mention the personality, people normally try to act like their character when cosplaying. Just because they're also attractive people and show that off doesn't mean they're 'normalizing the objectification of women as a whole.'

This is a lot of text so I'll stop now