There have been a lot of rrats and theories thrown around since the LLD, and as usual many of these have been absolutely moronic.
So let’s set the record straight in regards to Mrs. Mimi Nightmare’s little lore drop.

The first major question most people have had is “why?”
Obviously it was unnecessary to say anything about her marriage at all, and will ultimately destroy her viewership and revenue. We are already starting to see this. So, why?
Nimi is too intelligent and too stable to have simply done it for 48 hours of virtue signalers on Twitter giving her fake congratulations. And she is not prone enough to women moments to destroy her own career just to own le incels and unicorns. So from this we can tell she must have an actual reason of some sort.

I have seen 2 rrats that seem to be realistic and possible, with one more likely than the other.
The less likely one is that she had heard a rumor, possibly from a trusted source, that her marital status was going to be leaked and that she wanted to get ahead of it.
While this is possible, I still have a hard time seeing her tank her own career over a rumor, and even if it was leaked she could have simply ignored it and kept a plausible deniability. This still is an actual possibly, but I think it is the less likely of the two reasons.

The second possibility is that her husband, in addition to demanding that the multiple month trips to Japan stop, also make her relationship to him known as part of their continuing relationship and marriage. Probably due to insecurity and jealously of having other men chatting with and hitting on his girlfriend.
Of course, this would be absolutely bullshit on his part, but we all know this kind of insecurity is actually extremely common. And so she probably knew she was going to make this statement at some point even before leaving Hololive.
If this is the case I honestly hope she gets a prenuptial agreement, because with that level of insecurity that marriage is going to last 2-3 years maximum. And as a multimillionaire she will be the one paying him in the divorce settlement.

In short, it almost certainly wasn’t for virtue signaling or a woman moment. And it most likely was something her fiancé demanded of her out of insecurity.