As a monkey, your duties are... Well. You don't have any. Literally, none. The upside to this is that you can basically do whatever the hell you want(EXCEPT GRIEF). But there are many, many downsides.
There are many ways to become an adorable chimp! The most simple and accessible is praying, the Gods always seem to have time to turn people into simeons. You can also ask the geneticist, but be careful, those folks are usually pretty shady.
As a monkey, you're free to do whatever the hell you please. Which is certainly fun. You also have the ability to crawl through vents to reach any other part of the station with a vent. Scrambling around as a monkey, and harassing the crew is a very fun thing to do. Otherwise, there's no real upside to being a monkey.
On the otherhand, there are many, many downsides of being a monkey. As a monkey, you essentially lose your rights of being a human player. The AI can do whatever it sees fit with you, whether it be make you touched a shocked door, and drain the air from your room. Same with cyborgs. Another downside is your inability to wear clothing. You CAN wear backpacks, and pick things up, but the disadvantage here is you are unable to crawl through vents with these things equipped.
A NOTE: Monkeys are much weaker than human beings and as such will more likely than not be knocked out by a singular regular punch to the head.
A SECOND NOTE: Your punches have been replaced with bites, which are useless in every way. Seriously, grab something else if you need to fight.
A THIRD NOTE: Usually, the other members of the crew mistake you for a changeling, which usually results to you getting whacked to death with a toolbox and the chef making burgers from you.