For me it is the fact that she is another "expert" with no expertise, not unlike me and many other people, but we don't get the publicity.
If she is aspergic, maybe that affects how she comes across in the media, I haven't seen much of her but she seems not to be able to countenance any view but her her own and I tend to turn off when I hear anybody like that whatever they are banging on about - they may be right they may not.
The climate is changing and we aren't helping it, that to me is certain. How much of it is down to human activity and what activity, is I think less certain and people who want to research that are less likely to get funding. "Greta's "Do the science." For me isn't an answer. Mind you there are worse than her, just stop oil, insulate Britain - take your pick and plenty on the other side as well.
Also I've not heard any solutions from her or many other climate activists other than ones that will cause a lot of hardship for a lot of people. (Again I may be wrong as I said I don't listen to a lot of stories about her) There needs to be a sensible look at the problem and the possible solutions and the effects of the "solutions" which may prove to just cause more problems in the future. We are set on a technological path if we abandon it lots of people will probably die. In my view we need to use the technology properly to try and resolve the issues.
I was born into a time when fewer people had cars, but public transport was safer, cheaper and better. When foreign holidays and air travel were not seen as a right. Food was seasonal and more was grown in this country. Milk, beer, and other drinks were sold in re usable bottles and people washed handkerchiefs and nappies. Blaming previous generations isn't helpful. I'm sure WWI and WW2 caused a lot of pollution and the Bloody Elisabethans chopped down all those oak trees to make ships. Every generation has made mistakes or come up with solutions that turned out to be wrong (Catalytic converters anyone?)
In my view (Another know nothing expert) we need to: incentivise reduction in population, Increase local food production. Provide efficient, safe, cheap public transport systems as people will use it if it is seen to work. Cut back on air travel and make more use of IT conferencing for business trips etc. on a small level re cycle glass by washing the bloody bottles not melting them down and making different bottles and I'm sure the same could be done with other packaging. On a larger level look at cheap ways to get into orbit and utilise space (free power, no gravity, all sorts of minerals) for heavy industry and take that pollution off Earth.
However most meaningful measures rely on those in power politically and commercially to make sacrifices and invest and it is far easier for them to tell me not to flush the loo as I'm killing the planet. I'm guessing that Greta and others like her are useful ambassadors for those solutions. My guess is that she is being exploited probably by both sides of the argument and if she isn't careful her lack of knowledge and experience is going to backfire on her and her cause and I'm not sure how she will be able to handle that. I respect her courage and the sincerity of her belief but I actually feel a bit sorry for her.