That is a regional peculiarity specific to the northernmost parts of Sweden, and, I guess, the northernmost Swedish-speaking minority of Finland. Note: Sweden is very unevenly populated; the overwhelming majority of the population lives in the south half of the country, where also its three major cities are located. The northern parts of Sweden mostly consist of vast stretches of wilderness, forests, mountains, and people who talk funny.
I think it's a little more common purely for fun, almost like slang, among some Swedish-speaking Finns in Helsinki (where she's from)."Ja" is still more common, but it's fun to inhale sometimes
https://en.wikipedia.org/wiki/Ingressive_sound gives some details about that. I knew that some Irish English speakers inhale when saying "yes", but it seems fairly widespread across Europe...
