[Chibi-Chan]

I wouldn't jump to the conclusion that the hate on the internet for Ayu has something to do with Japanese culture. Britney for example gets similiar hate posts since basically 15 years now.
I think the answer to why those people make those comments, that are mostly about their bodies and per se horrible voices is simple: they are women. The world has a lot of hate for every female who dares to not look as fuckable as she could and furthermore people want from both that they sing with a voice that isn't their natural tone at all but makes them sound more like young girls, so that's basically the first reason again. Since somehow only women who look and sound like girls are fuckable and therefor worth noticing and allowed to take up space, but not women who passed 30 and dare to be almost 40.
This is a culture we all grew up with. People here constantly also comment on how she looked bloated, fat in a paticular dress or whatever. Even though they are not directly bashing her and don't want to be mean or something, it's part of a culture in which it's totally fair game to analyse women's bodies and to comment on them. We honestly don't need to wonder, that if we think we have the right to analyse Ayu's body and to comment on it, that people who don't like her basically do the same, only without trying to be understanding like members here try to be.
Personally I think we should stop commenting on how her body looks. Because everytime we say she looks hot, we still judge her and automatically "save" this version we like in our brains and need to find an explanation when she doesn't look as we expect her to look. There is no "positive" judgement of women's bodies, because when we judge that this woman is hot we automatically set a standard women have to fulfill.And then the discussions about Ayu having her period or using fillers start.

(Edit: I don't think saying something like "she looks good" or "she looks sexy" is automatically the same as saying "she looks hot". Looking good or sexy can mean a lot of things, whereas there are no two ways of looking hot within celebrity culture and I think there are a lot of damaging iimplications which "looking hot" has for women, because it's way more about physical appearance than about different things like overall style, charisma and so on.)