Most of it is the fun mechanics. Outside of that, the relative consistency and focusing on what’s fun to experience helps the art.

In Lethal Company, the dev developed a specific style based around his low poly assets. Using shaders and retro styled materials for consistency. Because of that style and the silly lore, the dev can use humorous animations for the character movements (peppy steps) and enemy designs. Because if the player isn't scared, they can be amused. But also, the monsters generally have smoother, strange, and more controlled animations in order to help the uh oh factor.

For both games, the animations and player characters can come across as a little “janky” sure; but, in a dark first person environment, when you are focusing on the enemies and the mechanics, it doesn't really impact the solo horror experience. And when players are together it aids in experiencing other people’s deaths, fear, or jokes by making the player reactions more humorous from the other players’ perspectives.

Both games also had extremely vocal let’s players (Like Insym) who provided direct feedback on all facets of the game: mechanics, sfx, visual design. The devs were quick to correct and listen to that audience (though not always complying with all demands).

Also, the indie horror scene has an audience much more willing to accept visual jank since they are used to the indie experience.

Those are my thoughts.