It is not an English accent - source: me, in England for a long time. Nobody sounds much like that.

She did have a period of not eating, or limited eating, so was likely undernourished. I don't think it was long enough to make her this way. Maybe she's just small.

She's got a sister, two years younger. Compare their faces here:

https://heavy.com/news/2019/09/beata-ernman-thunberg/

The sister looks the same height and has a normal face. It emphasises that there is something wrong with Greta. If anything, it makes her look about 2 years older than Greta. Greta has a rounder face, squinty eyes, thin lips, no chin. There have been a lot of speculations about the cause of Greta's looks, but it is especially enlightening when we compare her to someone with 1/2 the same genes that we can see what she should have looked like.