I mean for the instantiation phase - not for generating derivations or building anything.
The Nix expression language, which is a pure lazy functional language with dynamic typing, is therefore closer to Jsonnet. Jsonnet differs from the Nix expression language mainly in that it also has object-oriented semantics in the form of mixins. Also, Jsonnet is an extension of JSON, whereas the Nix expression language is not (in particular it lacks floating point numbers). Finally, the Nix expression language is an embedded part of the Nix package manager (and other tools), whereas Jsonnet is a standalone evaluator that can sit in front of anything that accepts JSON.