Should I be feeling this uncomfortable to post whatever on Facebook?

I have cousins who all act old and are so proper over what they post. I know it's important to work I work and go to school too but that's all they talk about… And there always with large groups of people. They probably don't even know what a selfie is, memes, or anything… I feel so uncomfortable posting certain things like my iPhone lock screen for example which is this: in total fear that I'm going to be looked at as a baby… I don't want their lives but I feel pressured into being like them. They have no sense of music, movies… All they care about is gossip and loads of friends.

So figure out how to block them from seeing what's on your timeline without losing them as friends.

I can only assume you are a young teen and you haven't found your place in life yet, and your cousins have.
SO what if they don't know what a meme is, they seem to be living their lives and getting on. You seem to be sitting watching it all happen around you.

Posting on Facebook isn't compulsory
the example you have posted would be ok if you are in deed 13 or 14, any older, yes it would look juvenile.
I'm not sure exactly who is making you feel pressured into posting, once you mature and grow up a little, you will have plenty of things to post about