Are Samsara and Nirvana just states of mind?
Yes, when you realize that all is mind.
No, when you identify with what you think the mind is - the thinking/feeling process inside the head of a human who lives in the outside world. This “mind” can indeed be in many states, and it can also experience a transcendental state, a blissful state, lots of states, lots of experiences - none of which is Nirvana.