If you have friends who you can rope into also doing it and don't have a lot of things to do then it can be worth it. To successfully complete the carnival weekly usually does not need any of the harder to get spells (occasionally you'll see why X spell might have made something a lot easier though).

You can powerlevel blue mage in an hour or so, and then get a bunch of the open world spells (check a guide) and you should be pretty well and set to complete the carnival as it goes and not be annoyed that you have a level 0 job on your list.

If you don't have friends and your PF on your server is low activity for blue mages then it might not be worth the time as you would probably be able to do other tasks better. I do the weekly carnival because it's easy, reliable, and solo - which means it's on my time when I want and I don't need to worry about much, sometimes you'll see a challenge type that you don't have a primal spell for or something and be like "skip" but that's fine imo lol.

In terms of fun or value outside of that, I don't think the Blue Mage is in a good place and I don't put it very high on worth in it personally (which makes me quite sad as I was one of the few asking for blue mage before blue mage came out).

Some other things you might get out of blue mage besides carnival is the ability to kill some fate bosses with the death spells (many are immune, some are not), and the ability to death whenever you want (you can death teleport in town for example, these are called blood ports by some of the older people here lol).