She's 16, she looks 10 because she has some kind of disease. Wiki says she stopped eating for a while due to her mental problems, that's an explanation of why she's shorter than 5'(Swedish women are around 5'6").
There is a common fallacy called an ad hominem argument. In other words, you shouldn't dismiss an argument because the person uttering it is bad, not an expert etc. Of course, that's in pure logic, in practice you shouldn't listen to bums.
About Thunberg, I don't really care she's not an expert, I was willing to listen to the argument and it's not an argument at all. She is just saying we should do something because global warming is bad.
I guess that's true but what? Let's all stop using fossil fuels? Let's stop modern agriculture and cultivate the fields with oxen? Let's just allow nature to take over?
It looks like a case of mass hysteria, a bunch of people are concerned about something, they don't know how to solve it and decided to follow a mentally unstable kid as their savior.