I am very new to role playing games, and to try and help get my RPG chops up, I thought I would write in a journal about my PC's adventures. However I am curious if this is a appropriate for a PC who has an intelligence of 12 or not. I would love some input if you have an opinion, or from people who do this.
I don't see why not, 12 is considered to be high average/slightly above average or actually equivalent to an iq of 120 (which is definitely above the average of 100) depending on who you ask.
You could also write what amounts to game session notes, which would just go over the important details of what happens each game session and doesn't have to be done in first person as your PC. I keep session notes as a DM and it is helpful to have to reference. Your DM might appreciate you keeping something like this (whether it is done in journal form or not) as well.
As a GM, I love it when players keep campaign notes.
As a player, I am now doing campaign notes for the Emerald Spire campaign that we're in, and am often taking notes of special lines and actions of the PCs, names and agreements made with NPCs.
The GM reads through the notes (made on google docs) so that he can keep track of what happened last time and plan things based on our prior actions.