Is conquest immoral? You'd be a hypocrite to think so, seeing as you benefit from the European conquest of the New World, the Roman conquest of Britain, and everything in between.
But to bring the conversation back to the topic at hand, in the case of Hawaii: yes.
Unambiguously the American legal and civil culture that eventually replaced the old monarchy is more moral. As judged by my western outlook that places high value on individual rights, the rule of law, and scientific inquiry and decidedly low value on idol worship that's not done in the privacy of one's mind or community but is imposed upon everyone else through demands for restrictions on the use of land not owned by the people making the demands.
"But RightwingNutjob," you might ask, "can't you see you've fallen into my clever trap? There you are basing your conclusion on the supposed superiority of Judeo-Christian values. You can't do that!"
Of course I can. You asked me about morals. I answered back in the context of my morality as I understand it, which comes from God. If you subscribe to a different set of morals, it won't convince you, but it gives you the answer you were looking for. In the West, you see, we can base our convictions on different theological or atheological grounds and co-exist so long as we stay out of each other's way. I doubt the same was true under the old monarchy in Hawaii.