Most of the times the data class members I use are immutable.
As Kotlin function parameters are immutable, data class members should be also by default, especially if you do lots of functional paradigm.
data class User(firstName: String, lastName: String, age: Int)
is much cleaner than
data class User(val firstName: String, val lastName: String, val age: Int).
I am not saying that mutable types are prohibited but leaving out the
val should be also possible.
Just my opinion.