Returning inconsistent data types is not a best practice.

Stringifying results is not a best practice. Use appropriate data types.

( I'm not sure if the kata is even worth improving at all though. )

Almost duplicate:
https://www.codewars.com/kata/rotate-array
https://www.codewars.com/kata/rotate-array-js

Edge cases with k == 0 and k == arr.length - 1 should be tested.
And k == arr.length too that probably should perform the rotation instead...

roate([1,2,3],2) => '3 2 1'

That's not a rotation.

If k is greater than the array length or less than a zero, return the array.

A string is expected instead.