I personally do not think Meursault killed her.
Maman's death is pretty central to Meursault's story I would say, and important details tend to be kept the same around Mirror Worlds (Rocinante, Heathcliff and Cathy, etc.), and the one time we saw a glimpse of Maman - in PrinceSault's story - she dies in La Manchaland, not from his hand, but he pretty obviously blames himself for it, in my opinion. I think Maman died from reasons unrelated or slightly related to him, and he blames himself for her death, hence "I killed her" here means "I caused her death".
I think Maman was sent to M Corp. Facilities much like the BookMaman was sent to the Home for Elderly Persons, and she died there. I speculated that it was from natural causes - but it's true that only rich people in the City die of natural causes, and Meursault is poor (couldn't support Maman at home, just look at his living conditions as described in the book, look at what he eats, he is not wealthy). Maybe she was killed by someone, maybe there was some sort of incident. It's hard to say at this time, but I do think her dying of old age (even if the age is "about 60" which is very small lol) could be possible if she's in a specialized facility and a Nest, and it would be book-accurate and not interfere with an important theme later on. Speaking of that:
A big, HUGE part of the trial is Meursault being blamed for Maman's death. They even compare him to a person who actually committed patricide and say that Meursault is pretty much worse, for killing his mother in a less literal but more contemptuous way, iirc. And the important thing is that Meursault didn't kill her, he even did all he could - put her in a place where she could be supported when he couldn't sustain her, visited up until the moment he felt she didn't want him around (again iirc), he did attend her funeral even if he didn't cry. This serves to illustrate how for the courtroom people, not conforming and not showing proper emotions/behavior is worse than actual murder, and him actually killing Maman would completely nullify that aspect. So, yeah, I think she died for other reasons but N Corp. blamed him for how he acted.
More about Maman as a treat, because I saw other theories like mine - I believe she was abusive to Meursault, which could maybe serve as a motif for the court to use to blame him, either pinning the blame on him and saying he actually killed her or saying something like "he had reasons to hate her, and he decided to show that hate by not doing what he's supposed to" as another way of pointing out his non-conformism.
Anyway, why I think they hinted at her being abusive. Firstly, Meursault said she used to scold him, and I don't think just that makes her abusive (parents have to sometimes forbid children from doing something) I think it strengthens the other points. Secondly, in the book, she's compared to Salamano, a man who abuses his dog but ends up missing it greatly when it runs away, paralleling how Meursault abandoned Maman at the Home for Aged Persons, and therefore the abuse part may be relevant as well. Also, a smaller thing, when Meursault and Maman were still living in one apartment, Maman "was always watching him", I can't quite gauge the meaning of that but it seems like she was ignoring him, since they weren't talking, but was also like, too watchful/intrusive? I don't know? But. Finally, at one point in the second half of the book Meursault says something like "I did love Maman, but everyone wished for their loved ones to die at least once". I don't think that's normal actually, it might be about how he felt burdened by her, maybe, but to me it reads more like he had some reason to wish her gone, again, perhaps because she was abusive to him.
In the book there's also a few minor things not specifically related to abuse but hints at how Meursault's current personality was formed: Maman is quoted as saying that "over time one gets used to anything", a statement that seems to be held by Meursault as well; Maman is said to not expect much of people, and Meursault doesn't seem to place expectations on others either; and, I don't know if this is related to Maman, but Meursault says he "used to have ambitions as a student but soon realized it was futile when [he] had to drop [his] studies", the word "soon" makes me think that he didn't lose his ambitions BECAUSE of dropping his studies, but because of something caused by that - maybe Maman scolded him for that too, killing hope in him? This feels too blaming things on Maman actually, I think it's more likely that he just abandoned his ambitions as a result of dropping out itself
I literally cannot wait for Canto X, because, despite saying all this, I still have no damn idea what happened to either Meursault or Maman and I NEED to find out.