[Matrixbeast]

Well, judging a book by it's cover is exactly what you got with with Ninninger. The designs, weapons, and minor blurbs all pointed to it being a wacky fun series (Keep in mind that people were actually REALLY hyped up for the silly zany series). They got exactly what they were told.
Shockingly, people got sick of the zany stupidity after 2 episodes.

Since everyone got what they were expecting with Ninninger, they thought they'd get what they anticipated with Zyuoh.

Concerning people's judgment on Zyuohger, it's not like anyone is wrong. Mario is cringe worthy in the first 3 episodes, Lio is pretty loud, their designs are still super basic, and ZyuohKing is still godawful (It's not even until the reveal of the unreleased Rhinos that made me consider buying one). It holds up rather well in my (And some other's) eyes, but it's not like the early detractors don't have unfounded problems.