In vast majority of the cases, you don't control the backend, so the serialization format must be universal and even if you do, you don't want to tailor it for the specific frontend and its serialization format. Therefore I'd suggest to rewrite this into a general key: value format, which should cover majority of the use cases.

There is no universal format for serializing ADTs. In Mint, they come in three formats:

type ADT {
  // With labelled parameters
  Labelled(name : String, age : Number, active : Bool)

  // With parameters (no labels)
  Parameters(String, Number, Bool)

  // Just a tag no additional values
  Empty 
}

So key: value can only cover one of the three formats. Having different serialization based on the format would be confusing IMHO.

Mint also have records:

type User {
  active : Bool,
  name: String,
  age: Number,
}

They do serialize to key: value format.

As far as I can tell no mainstream language which have ADTs have automatic serialization for them, and even if they do it will never be 100% compatible. The point of this feature is to allow persisting and loading back Mint data easily.