96 year old mom is usually miserable. What might help her?

You say she is usually miserable. That leaves an opening. You didn’t say always miserable. What are the circumstances where she’s not miserable? Was it a different time of the day when you visited or called? Was the weather better, were you wearing a smile, brought her a rose or told her a funny story? Maybe you didn’t fuss that day or bring up a sore subject? Sometimes one day is better than another. Does someone always worry when you are out of sorts?
‘Make for yourself a check list before a call or visit. Don’t arrive with a to do list on parade. (thats what I regret}. Ask after her health and those she communicates with that you don’t. Pay her a compliment. Tell her a joke. If that doesn’t work, ask for a smile. That’s one of my favorite memories of my mom near the end the sweet smile given upon request. Hard to do on a regular phone call. Maybe a FaceTime? Seek her advice. How did you keep your crust from becoming soggy when you made your cobbler? DH is becoming forgetful. How did you handle that with dad? Make sure she feels better after your visit than before. Sometimes she might need a minute to unload all the ‘stuff’ before she can enjoy herself. Allow her that. You have agingcare for that. She probably doesn’t have that type release. All there is in this life reduces to love or fear. Give her a little love. That will help you both.

My 90 yo mom is the same. She's never been a particularly 'happy' person, but aging is hard, as I am coming to see myself.

Depressed, severely, for most of my life, she proudly never 'needed' antidepressants. Well, 5/6 of her kids did, that's a bit telling. At 90, she told me the other day she is totally content to live out her life in within her '4 walls'--which is her small apartment in YB's house. Basically told me to quit trying to clean for her, her house didn't get dirty (!) and she didn't want ANYONE touching her things.

I can respect that, and do. No more trying to 'better' her life. She'll probably live 10 more years, She's already outlived one of her children, I wouldn't be surprised if she outlives more of us.

Really, there is nothing I can do, and nothing I plan to do for her.

Nothing has helped my almost 94 y/o mother's chronic misery for her entire life. Nothing. She likes being miserable, I'm convinced of it, so she creates drama to have something to BE miserable about. If it doesn't exist, she will create it herself. Furthermore, she doesn't want 'solutions' to her invented crises, so all the hoops I've jumped through for decades have been for naught.

So, without something more than a 'header' with no details whatsoever, nobody can really assess your situation and give you helpful advice. You may want to expand on your question.

Has she been evaluated by her doctor for the depression and given meds to help? My dad was gently helped by his doctor with this, and a small dose of Zoloft was a help to him. You may also have to accept that “happy” is over for your mother. At her age, she’s experienced a lot of loss, of her family, peers, friends, her abilities, all that makes it very understandable to feel down. Acknowledge it with her, and then try to not endlessly loop back to it. Encourage positive conversation and topics

You know what they say about old age? That it isn't for Sissies? Very seriously it is very difficult. At 96 even the minimal body changes are causing problem with so many systems, and what honestly is chronic pain. You don't say what exactly the problem is, so I am going to ask you to ASK YOUR MOM what is happening, and what is the worst problem for her right now.
Then do come back and tell US, and we can all troubleshoot together.
I will ask you the one objective question I can think of. Was your Mom a whole lot different in the past, a happy, satisfied, life-affirming woman? And when would you say the change, if change there is, occurred?