I'm slightly revising what I said earlier. Part of my impression of Greta probably being a prodigy is that her small size gave me the impression that she is much younger than I later realized she is. Yesterday I learned that she is now 18 years old (see https://en.wikipedia.org/wiki/Greta_Thunberg) despite being small.
Regarding how smart she is, the Wikipedia article says "She finished lower secondary school with good grades.[65] In July 2019, Time magazine reported Thunberg was taking a "sabbatical year" from school ... " and it says that "In 2019 she completed the 9th grade (the completion of lower secondary education in Sweden) with excellent grades, 14 As, and Bs in three subjects:[36] Swedish, home economics and physical education.[37]" She is thus smart even if she is not a prodigy.