Are they though? Aren’t DTO’s supposed to be pure data objects? [...] So what about extensions?

I'm not 100% familiar with Kotlin, but I think extension methods in Kotlin are the same as C#, basically syntactic sugar.

So you'd write an extension method like:

public static class ObjectExtension
{
    public static void DoSomething(this object input) { }
}

this being the keyword making it an extension method - and you can call that function on an object by doing object.DoSomething() - Yes. But underneath it's the same as doing ObjectExtension.DoSomething(object) (A static invocation to your ObjectExtension class and DoSomething method.

So on the surface it looks like you're injecting a new method into your DTO, and your DTO is not a pure data object anymore, but actually you're just creating an helper function that's statically invoked - that looks like you can call it "on the object" but actually you're not.

As for whether it's a good / common practice to create mappers like that in Kotlin, I don't really know. I do it often in C# though.