Just as an update to this, I've just checked Bandai Japan's
financial highlights report for the last completed financial year, and also calculated the average of the episode ratings listed in
TV Nihon's wiki page for Ninninger.
Ninninger's total sales were ¥7.8 billion (yes, it just beat Gekiranger in the end and so is
not "the worst selling season since Timeranger"). It's average viewership rating was 3.9%, the same as Kyoryuger and a slight improvement over ToQger.
Just for a little bit of context, the same Bandai report listed that Kamen Rider (roughly the last half of Drive and first half of Ghost) had sales of ¥15.7 billion, but Ultraman only had sales of ¥2.7 billion.
Sentai from Jetman to Ninninger, ordered by merchandise sales, highest to lowest.
Oh, Kyoryu, Zyu, Hurri, Aba & Gokai (joint), Go-On, Car, Deka, Jet & ToQ (joint), Gao, Magi, Shinken, Bouken, Go-Bus, Ginga, Gosei, GoGoV, Kaku, Dai & Ninnin (joint), Geki, Mega, Time.
Sentai from Jetman to Ninninger, ordered by average viewership rating, highest to lowest.
Gao, Ginga, Magi, Hurri, Jet & Zyu & Time & Aba & Deka (joint), Dai, Bouken, GoGoV, Mega, Shinken, Kaku, Gosei, Geki, Go-On, Gokai, Car, Oh, Go-Bus, Kyoryu & Ninnin (joint), ToQ.
Please note that where two or more shows are listed as "joint", they had matching values and so I have put them in chronological order. We'll never get actual numbers to know what order they should be in.