Tough. If it is horror games in particular you want to not see, that is trivial. Use a bookmark to access your favorite searches.
https://itch.io/games/genre-rpg/tag-pixel-art?exclude=tg.horror
If it is all thumbnails you want to blur, that is kinda trivial too.
@-moz-document url-prefix("https://itch.io/games") {
.game_cell img{filter: blur(7px)}
.game_cell img:hover{filter: blur(0px)}
}
Put this into a user style extension as a style. It will blur all thumbnails, until you move your mouse over them.
Preemptively blurring/filtering tags is a problem, since the tags are not displayed on the page in most cases. (Main) Genres are possible.
But blurring is probably not helpful. It might create new simulacrum images, so you would see faces, where there are none.
@-moz-document url-prefix("https://itch.io/games") {
.game_cell img{rotate:180deg}
.game_cell img:hover{rotate:0deg}
}
Maybe another image transformation might help, like {scale: 0.3}
If you are just bothered by the images but can stand selecting them, so you do not see the game next time you browse, I suggest looking into the third tool on that project page linked above. You just press a key with the mouse over a game and it will disappear. For filtering I consider it far superior to tag exlucions.
Tags are unreliable. If you filter out horror, your still get psychological horror where there might be more distored images and you cannot be sure if developers tagged both.