But others cast doubt on the announcement, with another Twitter user saying “this timing means ‘don’t criticize.’”
Mako is sister to Prince Hisahito, 15, currently the only eligible male heir to the throne other than his father.
Japan’s Chrysanthemum Throne can pass only to male members of the family, and the children of female royals who marry outside the aristocracy are not included.
The strict rules have raised fears for the future of the royal family, with just two male heirs and no prospect of new ones for years to come.
But attempts to debate succession reform have been met with staunch opposition from traditionalists.
Mako is not the first member of the royal family to suffer the consequences of hostile media attention. Empress Masako has long battled a stress-induced illness that emerged after she struggled with the pressure to bear a son and royal restrictions on travel.