I was nodding my head at your theory until it fell off a cliff here...I've seen enough Iris comments to able to form a legitimate theory on why people dislike her.
Fact: most people dislike her because of the way she treats Ash. Those same people then decide to find ways to crap on Iris because they dislike that, so they come up with BS arguments for why she's the worst character ever, when she's one of the better written characters in this series.
Haters gonna hate, I guess.
So why does Japan dislike Iris?The poll was conducted by the Japanese site AnimeAnime, also I have doubts that the reason every gave is the reason she was last because she was a Japanese site and the reasons you gave are the Western audiences reasons.