Some posts are being hidden possibly due to the rating system of people voting down ones they dislike, is it possible to remove the voting system or to stop posts from being hidden? It's quite annoying to do a search, get 1 result but not being able to see it without manually clicking show hidden posts.

If people don't want to see lewd posts they should use rating:safe or add the lewd tag to their blacklist.

Okay turned off anon voting.
Do you have example search?

https://steamcovers.booru.org/index.php?page=post&s=list&tags=lewd gives 3 hidden results, searching for undertale has 1 hidden result (all results hidden), is there a reason for voting/negative voting/hiding to exist at all? I don't see any positive effects from it but the negative ones are obvious.

Looks like you can't disable it booru wide for everyone.
https://forum.booru.org/viewtopic.php?f=2&t=242
You can set your account settings though.
When I have a second I'll try and set up a script to detect and fix/unhide any downvoted images.