Your question isn’t really Kotlin vs. Flutter as one is a language and one is a platform. The proper question could either be Kotlin vs. Dart, but more likely, Android-only native development vs. cross-platform development with Flutter.
As someone fluent in both I think Flutter is definitely the future, and it is mature. However the adoption rate is low so finding a job in it is hard. I think developing the same app twice with 2 different languages and toolsets is just not a model that scales and that is where Flutter shines, but most of the world seems to be happy doing 2 versions of the same app.
As far as languages I can say that Dart is not a bad language and is “good enough” but coming from Kotlin it is a let down and you will get frustrated with simple things like needing to put in semicolons and the lack of extension functions or some of Kotlin’s nice lambda syntax sugar.