In Japan they have a little something called CERO:

Smash Ultimate there is rated A so That’s why Mai wasn’t added

As dolanrules elaborated, yes, Japan's rating board is far more strict than the ESRB. Like, yes, they allow some things that the ESRB does not, but at the cost of increased rating, a risk that normally no developer ever takes, and CERO is very uptight about it, unfortunately.