As is visible in the screenshot below, my Sim has all their needs met and no negative moodlets, yet their happiness level is pretty much stuck to miserable. Is this a bug, and if so, how do I fix it?
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Supposedly this is due to a bug that causes the well rested moodlet to apply a permanent negative mood effect, instead of a temporary positive one. At least this is according to this fix. I use this fix and it does seem to make the mood work correctly.
To add on to Xantec's answer:
EA managed to totally screw up the "Well Rested" moodlet with the LN patch. For every sleep cycle sims would get a semi-permanent mood impact in the amount of buff value minus 25. So for the cheapest bed, the mood would drop by 15 per day. The "Well Rested" moodlet is only the most obvious case by the way, but not the only one. This mod will fix that issue for you.
So it looks like your Sim just has a really, really, depressing bed. Either buy a better bed, or download the mod Xantec linked to to fix it.
I had the same problem, I can see in the picture that your sim is not fullfilled (mood effect box). I believe that's the problem.
If your sim hasn't been fullfilled lately, they seem to get depressed with out it showing up in the mood effects box.
Try to fullfill your sim as much as possible and you probably won't have the problem again.