I think numbers have decreased overall for general solo artists, even big names like Ayu, Namie and Kumi. Honestly, I don't even know who the most popular (female) solo artist in Japan currently
is 
, especially not out of the newer acts.
Last year the highest-selling single by a solo artist on Oricon was Hoshino Gen's
Koi, which ranked in at #32 (Generasia says it sold 190k but lol). For comparison, MM。15's Tsumetai Kaze to Kataomoi / ENDLESS SKY / One and Only was #39. The highest-selling single by a
female solo artist last year was Fujiwara Sakura's
Soup, which ranked in at #71 (which apparently sold 76k according to Generasia). The only other female solo artists in the top 100 singles of 2016 were Amuro Namie at #75 (72k sold), Mizumori Kaori at #78 (69k sold) and Namie again at #89 (62k sold).
A LOT more female artists ranked in the top 100 albums sold for 2016, but many of them were compilations from popular artists such as Kyarypamyupamyu, AI, Nakajima Miyuki and ayaka. Though, JUJU's Snack JUJU〜Yoru no Request〜 sold 103k and ranked #39, and that was a cover album of all things. Utada Hikaru ranked #3 with 569k sold but it was her first release since 2008.
Albums seem to be the best bet, although I don't know if they're just less affected by Oricon's rules than singles are. And that's not including, say, digital sales.
I guess artists like AI and JUJU would be considered the most popular at the moment? I dunno. I'm not familiar with the solo artist scene at all. Solo female idols most definitely aren't as popular as some other genres, unless you're an 80s idol Japan still loves, like Matsuda Seiko, lol.
Also, Ariana Grande's Dangerous Woman ranked in at #49 with 82k sold and Adele's 25 was #69 with 63k sold

. Which is pretty amusing to me.