The answer to this really depends on what prism you are viewing it from?
As a globe, we are pretty opposed to imperialism these days. China has started to dabble a little more in the South China Sea but we mostly have our nation states and there isn't really a lot of movement.
That was not the case at the time of annexation.
So, is it ethical to subsume a sovereign nation? No
Was it strategically justifiable at the time? Yes. If the United States had not annexed Hawaii, the Empire of Japan likely would have stepped in.
Hawaii has enormous strategic value and was then justifiable at the time of annexation.
That being said, and I hope this is an obvious statement, imperialism is not a good thing and is ethically problematic.
