This gets me thinking (although there is not much to divine from three letters). Maybe it's a Jennifer Lawrence GIF-inspired "Yeah, right" then? Downplaying the significance?

To some online sources, it is used for both.

> "Yah": slang spelling of the word "yeah" (which of course can also be used ironically)

> Merriam-Webster: "Yah": used to express disgust, contempt, defiance, or derision; probably imitative of the sound of retching