I wholeheartedly agree with Kelly's point of view: if journaling is about putting yourself onto the page through words and/or art, then you can't ignore the bad stuff. It's part of life, so (I think) it should go into my journal. In my case, it doesn't always go onto the page as words (or at least not readable words)...
I also do NOT think art journal pages have to be 'pretty' or artistic' when they're finished; that's way too much pressure for me to put on myself! I know some people use their art journals as a place to practice techniques and try out new things (like color combinations) but I don't do that. My journal isn't practice for other artwork, it is the finished product. So, not every page is going to be 'artistic' or 'pretty' when it's done. It took me a long time to be okay with that, but now, it's a place to do what I want-- if that's pretty in the end, so much the better, but that's not my goal.
Here's a couple of my recent journal pages as an example:
This page was done after a bad day at work-- I needed to get my bad mood out of my head, so I used Sharpie paint pens to do layered writing, so the actual words aren't readable, but the overall 'mood' comes through. It may end up as a background, or it may stay like it is. It's not pretty- not at all- but it shows how I felt, and I felt better after getting it out onto the page.
This page isn't about a bad mood, or being sad, but it does show that I don't always go for the 'artistic, pretty' page. I made this page after going shopping last Saturday and got some cool new stuff, so I wanted to document that on a page. I used scraps of all the papers I bought, and four different pieces of deco tape-- I know the things on this page don't 'match' each other, but that wasn't the point of it.
Here's an earlier journal page, from 2008. It's another one that's not pretty (in fact, the colors I used are actually kinda ugly), but looking at the topic, I'm pretty sure I wasn't feeling much like using pastel colors and flowers at the time...
all this is to say, journals are a personal space to do what you want, however you want to do it. Sometimes I do like 'pretty' and 'flowery' but not always-- and that's okay, because the only person I have to please in my journal is me!
and if you journal, the only person you have to please is you-- so go write some bad words in your journal if you want to, paint with some icky colors if they suit your mood, or scribble over something if you feel like it! (...seriously, scribbling is such a good way to get out your feelings, and it's fun to do-- lol!!)