Yaml, Toml, as well as Json, are usable in Hugo, as a front matter or data format.
Why not, AFAIK, hjson, although there’s a Go language hjson-go package?
What would be required to use it instead of the other format?
Adding additonal formats require lots of work (mostly maintainance, test, documentation) and would require really good reasons.
Lol, just learn json.