Yes, the converter gets you eighty percent of the way there. Usually it is a bit too conservative and e.g. biased towards making all types nullable because it is hard to reason about nullability in Java. In the same way it attempts to turn any method starting with the prefix get into a property, which does not always make sense when it is clearly a bit of business logic instead of a property. Finally, it seems to slab a lot of redundant generic type hints on things, which don't always work. As soon as you remove them, Kotlin's type inference usually gets it. So I typically spent a minute cleaning up after converting a file.
After that you get to the idiomatic stuff like e.g. making properties read only getting rid of multiple constructors by introducing default parameters. Getting rid of the builder pattern (mostly redundant in kotlin), introducing data classes where that makes sense, using lateinit vars to make nullable vals nonnullable, etc. Technically you are at that point improving things.
The cool thing is that you can convert to Kotlin on a file by file basis.