In my campaign, I am thinking about implementing an object that is cursed. What creature or higher being should curse it (not something too powerful, because the object itself isn't of much value)? Preferably a plant or fey, but it could be anything. The being that is cursing it will try and convince whoever is holding the object to do bad deeds.

Low level fiend perhaps, seems exactly their area of expertise.

An undead such as a specter or ghost, some  devil or demon or a contruct that can shapechange like a Transformer! ☺

Ghosts and fiends are overplayed IMO.  Have the object be a far realm entity.

Puck.

that is, a minor fey noble. Created a pair of them, one to encourage bad deeds, one to encourage good deeds. He is trying to understand this whole “good and evil” thing the poor things are always blathering on about.

he will half assed haunt the wielder at unexpected times, and will mutter on occasion about how he wants to look at this thing called morality next (virtues and sins).

Puck is most famous as a trickster fey featured in Shakespeare, but has a long history as a mischievous little guy, also known as Robin Goodfellow.

Maybe if the item is crafted from wood, the wood was chopped out of the heart of a dryads tree. Now the spirit of the dryad haunts the item and whispers in their ear, trying to get the attuned user to exact revenge against civilization. It desires the user to attack people who travel in the wild and destroy marks of civilization like farm land, houses, towns, and other man made objects, and may grant the user additional nature based powers if they do so.

The spirit of a satyr who dared to insult a feylord. He was killed and his spirit bound to a musical instrument. He loves music, and tries to drive the user to use his charmed magical songs to hurt people, such as charming them and using them to your advantage, or causing them to dance until they die of exhaustion. He also tries to persuade you into doing mischievous yet harmful acts. 

A fiend that desires its blade to constantly be soaked in blood.

An elemental being that desires to see its element spread, regardless of the safety of society. So they encourage the attuned user to cause floods and landslides, start wildfires, and destroy wind barriers that prevent gales from tearing through and destroying land. 

A spirit of a dead dragon who drives the attuned user to greed and destruction. Causing them to hoard valuables and kill any who come in their way of their riches.

I think Mechanus is under used, and the idea of a neutral curse is fun to me. Imagine the item being cursed with Law and bad things happen when you behave in a chaotic manner, but maybe you get a boon the more lawful you act. See what loopholes your players exploit, since mechanus surely embraces technicality. 

A hag, maybe. They’re always cursing stuff. 

A very angry Myconid.

When I started typing, that was a joke, but the "curse" could be spores that influence the user. Maybe a bit like the zombie ant fungus; it wants you to get up high so you can spread it around.

But for general bad deeds influenced by a low-level fey or plant (i know myconids are fungi but their type is plant; blame d&d, not me)? A hag's probably your most obvious route, yeah.