Relationship bar loss for fairies absorbing positive emotions is too strong.

I turned my farmhand (so an NPC) into a fairy, and she is despised by all but 1 household member. She just keeps draining people and the one I had befriended her just balances out, because they also keep up positive socials.

This only works if you are directly controlling the fairy. Non household fairies automatically default to emotion absorption when they come visit you. The relationship bar loss shouldn't be so severe when it's a fairy friend or lover dropping in to say hi.

I am not opposed to using cheats as I do so a lot of the time myself, but that's what I mean when I say, "gaming the game". It's harder to keep up normal relationships with fairies compared to regular sims because the negative impact from absorption is a bit too strong.

You can replenish emotional force on a fairy house, or by drinking a potion bottled from the Moondrop Springs. You don’t actually have to absorb positive emotions from other sims.

FeartheGreat​ if you're not opposed to using cheats, the Beloved Trait usually stops relationship decline. You can add it with this cheat:

testingcheats true 

traits.equip_trait trait_Legendary

I add this to all my Sims now to stop relationships from declining. My current Sim is friends with one fairy and it works for their friendship but I haven't played as a fairy to see how their gameplay affects it. If you don't want to use a cheat you just need to complete the Friend of the World aspiration and a quick way to do that is to buy a Sim the Incredibly Friendly reward trait for 2000 points before starting the aspiration. 

Note: On console, cheats requiring testingcheats true will disable console achievements and trophies for that game save.

tbh I feel the same about there not being a way for human sims to offer vampire friends their plasma.