You can post anything
One of the sweetest things about writing a fiction journal, it's that your protagonist is, also, the supposed author of the whole thing. So, you can just follow the flow of the story and let your character write whatever and anything.
Let's take my own twelve-year-old Martin. Most often he simply summarizes the last events of his eventful life. School can be stressful enough, then there are sports, friends, not-so-friendly bullies, etc.
But often he just want to share his hobbies, because, why not? So, he'll tell about the conlang he's making up, his dungeon maps, and a little bit of coding. Yep, my boy, is of an odd kind: the wannabe nerd.
And, let me tell you. As I started to write the thing, I hesitated to include anything that didn't belong to the “main thread of the story.” But, Martin's my cherished boy, so I let him have his way.
And the end result is that I pretty much feel that it works. Showing his little things help to create a more rounded character and opens new threads for the narrative.
I was about to add a third thing, but, our AI overlords are doing that all the time; so I shall refrain.
However… going to our usual blogging, there are some things that are generally more popular than other things. For example, I've found that literature is less popular than non-fiction —as posts in a blog. That's little surprise because you come to blogs looking for non-fiction. And then there are subjects that will get you more views than others.
Still, if you want to wrote about Ediacarian sponges, or your other-than-D&D tabletop fantasy role-playing game, well, do it! It still “belongs” in a blog, and it's a precious thing to have.
But yeah, it won't be as popular as the current trendy thing everybody is talking non-stop right now.