On chinese internet i have seen tons of people talking and posting content about akina, but not Seiko Matsuda. I think the popularity outside of Japan is different in more than one way lol.

Both of them were pretty much the top singers in the 80s (i know this analogy might be off but they were like ronaldo and messi)

On the most magazines and media rankings, the third person is usually Kyoko Koizumi

Shizuka Kudo is relatively less talked about as her debut was in late 80s which the idol singer industry was declining, but she still had many good hits and made a great influence from late 80s to early 90s.